pinctrl: samsung: Explicitly cast pointer returned by of_iomap() to iomem

pinctrl: samsung: Explicitly cast pointer returned by of_iomap() to iomem

For S5Pv210 retention control, the driver stores the iomem pointer from
of_iomap() under a void pointer member.  This makes sparse unhappy:

    drivers/pinctrl/samsung/pinctrl-exynos.c:664:36: warning: incorrect type in argument 1 (different address spaces)
    drivers/pinctrl/samsung/pinctrl-exynos.c:664:36:    expected void const volatile [noderef] <asn:2>*addr
    drivers/pinctrl/samsung/pinctrl-exynos.c:664:36:    got void *

The iomem pointer is used safely (stored under priv by
s5pv210_retention_init(), used by s5pv210_retention_disable()) thus we
can add explicit casts to iomem to silence the warning.

Signed-off-by: Krzysztof Kozlowski <krzk@kernel.org>