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/juno/include/platform_def.h b/plat/arm/board/juno/include/platform_def.h
index ddbc9b7..e096e33 100644
--- a/plat/arm/board/juno/include/platform_def.h
+++ b/plat/arm/board/juno/include/platform_def.h
@@ -222,6 +222,7 @@
 
 /* MHU related constants */
 #define PLAT_CSS_MHU_BASE		UL(0x2b1f0000)
+#define PLAT_MHUV2_BASE			PLAT_CSS_MHU_BASE
 
 /*
  * Base address of the first memory region used for communication between AP