imx: move get_boot_device to common file

i.MX8MN/P/ULP supports ROM API, they have almost same get_boot_device
implementation, so move to a common file. And when support i.MX9,
no need to include the other function copy.

Since sys_proto.h is included in imx_romapi.c, there will be build
warning for i.MX8M because wdog_regs not defined, so include imx-regs.h
in i.MX8M sys_proro.h

Signed-off-by: Peng Fan <peng.fan@nxp.com>
diff --git a/arch/arm/include/asm/arch-imx8m/sys_proto.h b/arch/arm/include/asm/arch-imx8m/sys_proto.h
index f8854e7..55b46af 100644
--- a/arch/arm/include/asm/arch-imx8m/sys_proto.h
+++ b/arch/arm/include/asm/arch-imx8m/sys_proto.h
@@ -7,6 +7,7 @@
 #define __ARCH_NMX8M_SYS_PROTO_H
 
 #include <asm/mach-imx/sys_proto.h>
+#include <asm/arch/imx-regs.h>
 
 void set_wdog_reset(struct wdog_regs *wdog);
 void enable_tzc380(void);