dts: vt8500: Update pinctrl nodes to use a platform compatible property.

Add an additional compatible property to the PRIZM platform pinctrl nodes
to allow us to match all the variations on a single compatible string.

Signed-off-by: Tony Prisk <linux@prisktech.co.nz>
arm: vt8500: Fix gpio mux setup in vt8500_init()

With the switch to a pinctrl driver, and the dropping of the gpio driver,
the code in vt8500_init now always fails, and drops back to LEGACY mode.

Update the gpio mux init code, and removing the #ifdef's and the LEGACY
mode fallback.

Signed-off-by: Tony Prisk <linux@prisktech.co.nz>