phy: exynos-mipi-video: Use consistent method to address phy registers

Exynos4 MIPI phy registers are defined with macro calculating the offset
for given phyN.  Use the same method for Exynos5420 to be consistent.

Signed-off-by: Krzysztof Kozlowski <krzk@kernel.org>
Reviewed-by: Bartlomiej Zolnierkiewicz <b.zolnierkie@samsung.com>
Signed-off-by: Kishon Vijay Abraham I <kishon@ti.com>