ARM: dts: imx7-mba7: Mark gpio-buttons as wakeup-source

I2C expander is capable of generating an IRQ during powersave, so the
attached buttons can be used for waking up the system.

Signed-off-by: Alexander Stein <alexander.stein@ew.tq-group.com>
Signed-off-by: Shawn Guo <shawnguo@kernel.org>