ARM: omap3: ti_omap3_common: Fix CONFIG_SYS_NS16550_REG_SIZE compiler warning

This commit fixes the following compiler warnings when DM_SERIAL is
enabled.

---8<-------------------------------------------------------------------
...

  CC      spl/board/ti/evm/evm.o
In file included from /solution/scm/u-boot-master/board/ti/evm/evm.c:16:0:
/solution/scm/u-boot-master/include/ns16550.h:31:0: warning: "CONFIG_SYS_NS16550_REG_SIZE" redefined
 #define CONFIG_SYS_NS16550_REG_SIZE (-1)

In file included from /solution/scm/u-boot-master/include/configs/omap3_evm.h:22:0,
                 from include/config.h:5,
                 from /solution/scm/u-boot-master/include/common.h:21,
                 from /solution/scm/u-boot-master/board/ti/evm/evm.c:14:
/solution/scm/u-boot-master/include/configs/ti_omap3_common.h:33:0: note: this is the location of the previous definition
 # define CONFIG_SYS_NS16550_REG_SIZE (-4)

  LD      spl/board/ti/evm/built-in.o

...

  CC      spl/drivers/serial/ns16550.o
In file included from /solution/scm/u-boot-master/drivers/serial/ns16550.c:11:0:
/solution/scm/u-boot-master/include/ns16550.h:31:0: warning: "CONFIG_SYS_NS16550_REG_SIZE" redefined
 #define CONFIG_SYS_NS16550_REG_SIZE (-1)

In file included from /solution/scm/u-boot-master/include/configs/omap3_evm.h:22:0,
                 from include/config.h:5,
                 from /solution/scm/u-boot-master/include/common.h:21,
                 from /solution/scm/u-boot-master/drivers/serial/ns16550.c:7:
/solution/scm/u-boot-master/include/configs/ti_omap3_common.h:33:0: note: this is the location of the previous definition
 # define CONFIG_SYS_NS16550_REG_SIZE (-4)

  LD      spl/drivers/serial/built-in.o

...
---8<-------------------------------------------------------------------

Signed-off-by: Derald D. Woods <woods.technical@gmail.com>
Reviewed-by: Tom Rini <trini@konsulko.com>

include/configs/ti_omap3_common.h

@@ -28,10 +28,12 @@
 /* NS16550 Configuration */
 #define V_NS16550_CLK			48000000	/* 48MHz (APLL96/2) */
 #define CONFIG_SYS_NS16550_CLK		V_NS16550_CLK
-#ifdef CONFIG_SPL_BUILD
-# define CONFIG_SYS_NS16550_SERIAL
-# define CONFIG_SYS_NS16550_REG_SIZE	(-4)
-#endif
+#if defined(CONFIG_SPL_BUILD)
+#define CONFIG_SYS_NS16550_SERIAL
+#if !defined(CONFIG_DM_SERIAL)
+#define CONFIG_SYS_NS16550_REG_SIZE	(-4)
+#endif /* !CONFIG_DM_SERIAL */
+#endif /* CONFIG_SPL_BUILD */
 #define CONFIG_SYS_BAUDRATE_TABLE	{4800, 9600, 19200, 38400, 57600, \
 					115200}