PCI: keystone: Invoke phy_reset() API before enabling PHY

SERDES connected to the PCIe controller in AM654 requires
power on reset enable (POR_EN) to be set in the SERDES. The
SERDES driver sets POR_EN in the reset ops and it has to be
invoked before init or enable ops. In order for SERDES driver
to set POR_EN, invoke the phy_reset() API in pci-keystone driver.

Signed-off-by: Kishon Vijay Abraham I <kishon@ti.com>
Signed-off-by: Lorenzo Pieralisi <lorenzo.pieralisi@arm.com>