can: grcan: add missing Kconfig dependency to HAS_IOMEM

On ARCHs without IOMEM support the grcan driver fails to link due to
missing iomem functionality. This patch adds the missing Kconfig
dependency to HAS_IOMEM.

Link: https://lore.kernel.org/r/20210309140424.3331010-1-mkl@pengutronix.de
Reported-by: kernel test robot <lkp@intel.com>
Signed-off-by: Marc Kleine-Budde <mkl@pengutronix.de>