slim_msm: fix LPAE builds (64-bit division)

We can't currently divide 64-bit numbers on LPAE builds. Currently,
slim-msm.c is using a physical address as part of a division operation,
resulting in the following linker error:

      LD      init/built-in.o
    drivers/built-in.o: In function `msm_slim_tx_msg_return':
    drivers/slimbus/slim-msm.c:432: \
        undefined reference to `__aeabi_uldivmod'

Fix this by casting the physical address to an unsigned long.

Change-Id: I396697158c57c80240ccd039006422afd684f47d
Signed-off-by: Mitchel Humpherys <mitchelh@codeaurora.org>