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/socionext/synquacer/include/platform_def.h b/plat/socionext/synquacer/include/platform_def.h
index 263b215..7e54b39 100644
--- a/plat/socionext/synquacer/include/platform_def.h
+++ b/plat/socionext/synquacer/include/platform_def.h
@@ -80,7 +80,7 @@
 #define DRAMINFO_BASE			0x2E00FFC0
 
 #define PLAT_SQ_MHU_BASE		0x45000000
-#define PLAT_CSS_MHU_BASE		PLAT_SQ_MHU_BASE
+#define PLAT_MHUV2_BASE			0xFFFFFFFF /* MHUV2 is not supported */
 
 #define PLAT_SQ_SCP_COM_SHARED_MEM_BASE		0x45400000
 #define SCPI_CMD_GET_DRAMINFO			0x1