omap: Fix AM335x build with enabled fastboot flash

When enabling CONFIG_FASTBOOT_FLASH in am335x_boneblack_defconfig, next
build errors and warnings occur:

    arch/arm/mach-omap2/utils.c: In function ‘omap_set_fastboot_cpu’:
    arch/arm/mach-omap2/utils.c:26:16: warning: implicit declaration of
              function ‘omap_revision’ [-Wimplicit-function-declaration]
              u32 cpu_rev = omap_revision();
                            ^~~~~~~~~~~~~
    arch/arm/mach-omap2/utils.c:29:7: error: ‘DRA762_ES1_0’ undeclared
              (first use in this function)

Include asm/omap_common.h explicitly to avoid those.

Signed-off-by: Sam Protsenko <semen.protsenko@linaro.org>
Acked-by: Lukasz Majewski <lukma@denx.de>
Reviewed-by: Tom Rini <trini@konsulko.com>

arch/arm/mach-omap2/utils.c

@@ -7,6 +7,8 @@
 #include <common.h>
 #include <asm/setup.h>
 #include <asm/arch/sys_proto.h>
+#include <asm/omap_common.h>
+
 static void do_cancel_out(u32 *num, u32 *den, u32 factor)
 {
 	while (1) {