ARM: dts: sunxi: Change node name for pwrseq pin on Olinuxino-lime2-emmc

The node name for the power seq pin is mmc2@0 like the mmc2_pins_a one.
This makes the original node (mmc2_pins_a) scrapped out of the dtb and
result in a unusable eMMC if U-Boot didn't configured the pins to the
correct functions.

Signed-off-by: Emmanuel Vadot <manu@bidouilliste.com>
Signed-off-by: Maxime Ripard <maxime.ripard@free-electrons.com>