phy: uniphier-pcie: Add SoC-dependent phy-mode function support

Since this phy is shared by multiple devices including USB and PCIe,
it is necessary to determine which device use this phy.
This patch adds SoC-dependent functions to determine a device using
this phy.

When there is 'socionext,syscon' property in the pcie-phy node,
the driver calls SoC-dependt function instead of checking .has_syscon
in SoC-dependent data. The function configures the system controller
to use phy for PCIe.

Signed-off-by: Kunihiko Hayashi <hayashi.kunihiko@socionext.com>
Signed-off-by: Kishon Vijay Abraham I <kishon@ti.com>