arm64: dts: imx8mm-venice-gw7902: fix pcie bindings

Update the pcie bindings to the correct dt bindings:
 pcie_phy:
  - use pcie0_refclk
  - add required clock-names
 pcie:
  - remove pcie_phy clock as it comes from phy driver

Signed-off-by: Tim Harvey <tharvey@gateworks.com>
Reviewed-by: Fabio Estevam <festevam@gmail.com>
Signed-off-by: Shawn Guo <shawnguo@kernel.org>