plat: imx8mm: provide uart base as build option

Some boards (f.e. Verdin i.MX8M Mini) use different UART base address
for serial debug output, so make this value configurable (as a
build option).

Signed-off-by: Igor Opaniuk <igor.opaniuk@gmail.com>
Change-Id: I988492ccecbc3f64a5153b381c4a97b8a0181f52
diff --git a/plat/imx/imx8m/imx8mm/platform.mk b/plat/imx/imx8m/imx8mm/platform.mk
index c0cb6c2..e28f739 100644
--- a/plat/imx/imx8m/imx8mm/platform.mk
+++ b/plat/imx/imx8m/imx8mm/platform.mk
@@ -51,3 +51,6 @@
 
 BL32_SIZE		?=	0x2000000
 $(eval $(call add_define,BL32_SIZE))
+
+IMX_BOOT_UART_BASE	?=	0x30890000
+$(eval $(call add_define,IMX_BOOT_UART_BASE))