stm32mp1: update boot mode management

- export the function get_bootmode() and reused it in spl code
- manage uart instance by alias (prepare v4.19 binding)
- solve issue on nand instance
- restore console for uart boot

Signed-off-by: Patrick Delaunay <patrick.delaunay@st.com>
diff --git a/arch/arm/mach-stm32mp/include/mach/sys_proto.h b/arch/arm/mach-stm32mp/include/mach/sys_proto.h
index 41d4b40..8b426c0 100644
--- a/arch/arm/mach-stm32mp/include/mach/sys_proto.h
+++ b/arch/arm/mach-stm32mp/include/mach/sys_proto.h
@@ -13,3 +13,5 @@
 
 /* return CPU_REV constants */
 u32 get_cpu_rev(void);
+/* return boot mode */
+u32 get_bootmode(void);