sunxi: SPL SPI: Introduce is_sun6i_gen_spi()

So far we were using the CONFIG_SUNXI_GEN_SUN6I symbol to select between
the two SPI controller generations used on Allwinner SoCs. This is a
convenience symbol to roughly differentiate between "older" and "newer"
generation of SoCs.

The H6 SoCs is the newest SoC so far, but is sufficiently different to
not define this symbol. However it is using a SPI controller compatible
to the "new gen" SoCs.

To prepare for H6 support, we replace the check for this single symbol
with an explicit function, which can later be extended.
For now we just return CONFIG_SUNXI_GEN_SUN6I in there, so this does not
create a functional change.

Signed-off-by: Andre Przywara <andre.przywara@arm.com>
Reviewed-by: Jagan Teki <jagan@amarulasolutions.com>