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>

This driver is found on RK3368 SoCs.

The Mailbox module is a simple APB peripheral that allows both
the Cortex-A53 MCU system to communicate by writing operation to
generate interrupt.
The registers are accessible by both CPU via APB interface.

The Mailbox has the following main features:

1) Support dual-core system: Cortex-A53 and MCU.
2) Support APB interface.
3) Support four mailbox elements, each element includes one data word,
   one command word register and one flag bit that can represent
   one interrupt.
4) Four interrupts to Cortex-A53.
5) Four interrupts to MCU.
6) Provide 32 lock registers for software to use to indicate whether
   mailbox is occupied.

[Jassi: Removed unused variable buf_base]

Signed-off-by: Caesar Wang <wxt@rock-chips.com>
Signed-off-by: Jassi Brar <jaswinder.singh@linaro.org>