MIPS: ingenic: gcw0: SPI panel does not require active-high CS

The NT39016 panel is a fun beast, even though the documentation states
that the CS line is active-low, it will work just fine if the CS line is
configured as active-high, but it won't work if the CS line is forced
low or forced high.

Since it did actually work with the spi-cs-high property, this is not a
bugfix, but we should nonetheless remove that property to match the
documentation.

Signed-off-by: Paul Cercueil <paul@crapouillou.net>
Signed-off-by: Thomas Bogendoerfer <tsbogend@alpha.franken.de>