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>
8 files changed