plat/arm: mhu: make mhu driver generic

MHU doorbell driver requires arm platform specific
macro "PLAT_CSS_MHU_BASE".
Rename it to "PLAT_MHUV2_BASE", so that platforms other than arm
can use generic MHU doorbell driver.

Signed-off-by: Masahisa Kojima <masahisa.kojima@linaro.org>
diff --git a/plat/arm/board/rdn1edge/include/platform_def.h b/plat/arm/board/rdn1edge/include/platform_def.h
index 2ca0dd4..8480c08 100644
--- a/plat/arm/board/rdn1edge/include/platform_def.h
+++ b/plat/arm/board/rdn1edge/include/platform_def.h
@@ -16,6 +16,7 @@
 #define CSS_SGI_MAX_PE_PER_CPU		1
 
 #define PLAT_CSS_MHU_BASE		UL(0x45400000)
+#define PLAT_MHUV2_BASE			PLAT_CSS_MHU_BASE
 
 /* Base address of DMC-620 instances */
 #define RDN1EDGE_DMC620_BASE0		UL(0x4e000000)