phy: exynos-video-mipi: Fix regression by adding support for PMU regmap

After the Exynos Power Management Unit (PMU) driver was converted
to the platform device driver in commit 14fc8b93
("ARM: EXYNOS: Add platform driver support for Exynos PMU") and
then PMU device nodes added to Exynos4 DTs in commit
7b9613ac ("ARM: dts: add PMU syscon node for exynos4")
the mipi video phy driver started failing probing, due to overlapping
memory mapped register region resources.

Now all the Exynos peripheral devices which have registers in the PMU
region are supposed to use the regmap provided by the syscon driver.
So support for regmap is added in this patch, this unfortunately
creates yet another indirection into that supposedly trivial driver.

The additional mutex is required because single register is used by
PHY pairs (they share bit in a register). An improvement here could
be to allow a PHY instance be created with a driver custom mutex,
which would then be common for each PHY pair...