ARM: dts: at91: at91sam9g25: fix mux-mask pinctrl property

There are only 19 PIOB pins having primary names PB0-PB18. Not all of them
have a 'C' function. So the pinctrl property mask ends up being the same as the
other SoC of the at91sam9x5 series.

Reported-by: Marek Sieranski <marek.sieranski@microchip.com>
Signed-off-by: Nicolas Ferre <nicolas.ferre@microchip.com>
Cc: <stable@vger.kernel.org> # v3.8+
Signed-off-by: Alexandre Belloni <alexandre.belloni@bootlin.com>