PM / devfreq: exynos-nocp: Select REGMAP_MMIO

This driver uses devm_regmap_init_mmio(), so select REGMAP_MMIO to avoid
build failure.

Signed-off-by: Axel Lin <axel.lin@ingics.com>
Acked-by: Chanwoo Choi <cw00.choi@samsung.com>
Signed-off-by: Rafael J. Wysocki <rafael.j.wysocki@intel.com>