ARM: dts: nomadik: Fix polarity of SPI CS

The SPI DT bindings are for historical reasons a pitfall,
the ability to flag a GPIO line as active high/low with
the second cell flags was introduced later so the SPI
subsystem will only accept the bool flag spi-cs-high
to indicate that the line is active high.

It worked by mistake, but the mistake was corrected
in another commit.

The comment in the DTS file was also misleading: this
CS is indeed active high.

Fixes: cffbb02d

 ("ARM: dts: nomadik: Augment NHK15 panel setting")
Signed-off-by: Linus Walleij <linus.walleij@linaro.org>
Signed-off-by: Arnd Bergmann <arnd@arndb.de>