pinctrl: mvebu: armada-xp: add spi1 function

The latest Armada XP datasheet documents that some of the MPP pins can
be used to access the second SPI bus, labelled 'spi1'. This commit
adds the corresponding pins in the pinctrl driver and its DT binding
documentation.

Signed-off-by: Thomas Petazzoni <thomas.petazzoni@free-electrons.com>
Signed-off-by: Linus Walleij <linus.walleij@linaro.org>