rockchip: px30: Fix build error

"result of '1 << 31' requires 33 bits to represent, but 'int' only has
32 bits [-Werror=shift-overflow=]"

This is treated as an error since commit 93c690eb ("Enable
-Wshift-overflow=2 to check for undefined shift behavior")

Only the actual errors are being tackled by this patch. It is up to the
platform to choose whether there needs to be further modifications to
the code.

Change-Id: I70860ae5f2a34d7c684bd491b76da50aa04f778e
Signed-off-by: Ambroise Vincent <ambroise.vincent@arm.com>