The pinctrl groups for SPI until now were also adding the chip selects in
the SPI pinctrl group.

This was causing a few issues, since a board was forced to use a random
number of chipselects, even though it might use one of these chip selects
for another pin.

The number of chipselects defined was also not the same from one group to
another because of different needs at the time these groups have been
introduced, resulting in no clear view from the board DTS on what exactly
is being muxed, which even might change in the future.

Solve this by creating different pinctrl groups for the chipselects and the
standard SPI pins (CLK, MOSI and MISO) so that we fix both issues.
Signed-off-by: Maxime Ripard <maxime.ripard@free-electrons.com>