mailbox: cmdq: variablize address shift in platform

Some gce hardware shift pc and end address in register to support
large dram addressing.
Implement gce address shift when write or read pc and end register.
And add shift bit in platform definition.

Signed-off-by: Dennis YC Hsieh <dennis-yc.hsieh@mediatek.com>
Signed-off-by: Jassi Brar <jaswinder.singh@linaro.org>