phy: Kconfig: Update config for Exynos USB DRD

USB DWC3 driver on Exynos platform does not work without its
corresponding phy driver. Hence make the PHY driver depend on
Exynos DWC3 driver and default it to yes to make things easier
for the end user.

Signed-off-by: Sachin Kamat <sachin.kamat@samsung.com>
Reviewed-by: Jingoo Han <jg1.han@samsung.com>
Tested-by: Vivek Gautam <gautam.vivek@samsung.com>
Signed-off-by: Kishon Vijay Abraham I <kishon@ti.com>