remoteproc: omap: Add comment for is_iomem

remoteproc: omap: Add comment for is_iomem

Address warning: "Function parameter or struct member 'is_iomem' not
described in 'omap_rproc_da_to_va'" with adding comment for is_iomem.

Reported-by: kernel test robot <lkp@intel.com>
Closes: https://lore.kernel.org/oe-kbuild-all/202502161648.WZWrFV7I-lkp@intel.com/
Cc: Andrew Davis <afd@ti.com>
Signed-off-by: Peng Fan <peng.fan@nxp.com>
Link: https://lore.kernel.org/r/20250217075858.616737-1-peng.fan@oss.nxp.com
Signed-off-by: Mathieu Poirier <mathieu.poirier@linaro.org>