ARM: sun5i: Rename A10s pins

The SPI2 pins on the sun5i PB bank are only available on the A10s. Rename
the A10s only bank so that it doesn't confuse people on the other SoCs
whose indexing would start at b.

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