phy: samsung: Add Exynos USB DRD PHY driver

phy: samsung: Add Exynos USB DRD PHY driver

Add DM driver for Exynos USB PHY controllers. For now it only supports
Exynos850 SoC. Only UTMI+ (USB 2.0) PHY interface is implemented, as
Exynos850 doesn't support USB 3.0. Only two clocks are used for this
controller:
  - phy: bus clock, used for PHY registers access
  - ref: PHY reference clock (OSCCLK)

Ported from Linux kernel: drivers/phy/samsung/phy-exynos5-usbdrd.c

Signed-off-by: Sam Protsenko <semen.protsenko@linaro.org>
Reviewed-by: Mattijs Korpershoek <mkorpershoek@kernel.org>
Reviewed-by: Minkyu Kang <mk7.kang@samsung.com>
Signed-off-by: Minkyu Kang <mk7.kang@samsung.com>