arm64: dts: allwinner: a64: Rename r_i2c_pins_a label to r_i2c_pl89_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 "r_i2c_pins_a" label is renamed to "r_i2c_pl89_pins" to fit our
current style. The node name "i2c" is also changed to "r-i2c-pl89-pins"
to match. The reason for the peculiar name is that the other option for
muxing R_I2C is on the PL0/PL1 pins, so the name has to mention the pin
numbers in addition to the pingroup.

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