accel/amdxdna: Declare mailbox register base as __iomem pointer

accel/amdxdna: Declare mailbox register base as __iomem pointer

Declare mailbox ringbuf_base and mbox_base as 'void __iomem *' to fix
sparse warnings:
  cast removes address space '__iomem' of expression
  incorrect type in argument 2 (different address spaces)

Fixes: b87f920b9344 ("accel/amdxdna: Support hardware mailbox")
Reported-by: kernel test robot <lkp@intel.com>
Closes: https://lore.kernel.org/oe-kbuild-all/202412310857.syD1sXXO-lkp@intel.com/
Signed-off-by: Lizhi Hou <lizhi.hou@amd.com>
Reviewed-by: Mario Limonciello <mario.limonciello@amd.com>
Signed-off-by: Mario Limonciello <mario.limonciello@amd.com>
Link: https://patchwork.freedesktop.org/patch/msgid/20250102212244.1586340-2-lizhi.hou@amd.com