ARM: dts: mmp3-dell-ariel: Replace SSP2 with spi-gpio

The firmware leaves the pins in GPIO mode. Until we have a proper pinmux
driver hooked on we just need to bitbang SPI. No big deal, this is just
used for the power button and performance is not important.

Signed-off-by: Lubomir Rintel <lkundrak@v3.sk>
Link: https://lore.kernel.org/r/20210121034130.1381872-12-lkundrak@v3.sk

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Signed-off-by: Arnd Bergmann <arnd@arndb.de>