rpi: Determine PL011/Mini-UART availability at runtime

Firmware on the Raspberry Pi family of devices can dynamically configure either
the PL011, Mini-UART or no device at all to be routed to the user accessible
UART pins.

That means we need to always include both drivers, because we can never be sure
which of the two serial devices firmware actually chooses to use.

Signed-off-by: Alexander Graf <agraf@suse.de>

include/configs/rpi.h

@@ -75,12 +75,9 @@
 #define CONFIG_MISC_INIT_R
 #endif
 
-/* Console UART */
-#if defined (CONFIG_BCM2837) || defined(CONFIG_TARGET_RPI_0_W)
+/* Console UART, can be configured dynamically in config.txt */
 #define CONFIG_BCM283X_MU_SERIAL
-#else
 #define CONFIG_PL01X_SERIAL
-#endif
 
 /* Console configuration */
 #define CONFIG_SYS_CBSIZE		1024