generic-phy: s/CONFIG_PHY/CONFIG_IS_ENABLED(PHY)/

Allow to disable PHY driver in SPL because it checks the CONFIG_SPL_PHY
variable for SPL builds.

The same change was done for usb by commit fd09c205 ("usb:
s/CONFIG_DM_USB/CONFIG_IS_ENABLED(DM_USB)/").

Signed-off-by: Michal Simek <michal.simek@xilinx.com>