arm64: dts: allwinner: a64: Rename uart0_pins_a label to uart0_pb_pins

The pinmux name and label for a specific function should denote which
pingroup it is on, or if there is only one option for the function, have
not enumerating prefix/suffix at all.

The "uart0_pins_a" label is renamed to "uart0_pb_pins" to fit our
current style. The node name "uart0" is also changed to "uart0-pb-pins"
to match.

Acked-by: Maxime Ripard <maxime.ripard@bootlin.com>
Signed-off-by: Chen-Yu Tsai <wens@csie.org>