ARM: dts: turris-omnia: Fix mpp26 pin name and comment

There is a bug in Turris Omnia's schematics, whereupon the MPP[26] pin,
which is routed to CN11 pin header, is documented as SPI CS1, but
MPP[26] pin does not support this function. Instead it controls chip
select 2 if in "spi0" mode.

Fix the name of the pin node in pinctrl node and fix the comment in SPI
node.

Fixes: 26ca8b52 ("ARM: dts: add support for Turris Omnia")
Signed-off-by: Marek Behún <kabel@kernel.org>
Signed-off-by: Gregory CLEMENT <gregory.clement@bootlin.com>