lantiq: VGV7519 - cleanup pinmux configuration

Cleanup the pinmux configuration by removing the unused spi node. Nothing is connected to the SPI bus on devices.

The stp_out pinmux child node covers the same GPIOs as the already used
stp group.

The same applies to the gphy-leds_out pinmux node and the "gphy0
led1" as well as "gphy1 led0" groups.

Signed-off-by: Mathias Kresin <dev@kresin.me>