mailbox: rockchip: avoid 64-bit division

The newly added rockchip mailbox driver causes a bug in
the ARM allyesconfig build because of a division of a resource_size_t
variable that may be 64 bit wide:

drivers/mailbox/built-in.o: In function `rockchip_mbox_probe':
:(.text+0x6614): undefined reference to `__aeabi_uldivmod'

This adds a cast to size_t, which turns it into a 32-bit division
in this case. This is safe because we know that we cannot possibly
map a resource that is longer than what a pointer contains, and
in practice it will be very short instead.

Signed-off-by: Arnd Bergmann <arnd@arndb.de>
Signed-off-by: Jassi Brar <jaswinder.singh@linaro.org>