pinctrl: at91-pio4: Add support for pinctrl config subnodes

Previously, in order for the `pinctrl-*` DT node properties
to be properly processed, the pinctrl's subnodes were limited
to only having the `pinmux` property as well as other additional
properties (slew-rate, bias-disable, etc.). Now, with this patch
the pinctrl driver is made to work similarly to the one from Linux.
It can now distinguish between one subnode and a subnode with multiple
subnodes.

Signed-off-by: Sergiu Moga <sergiu.moga@microchip.com>