arm64: dts: imx8mq-librem5: Bump BUCK1 suspend voltage to 0.81V

0.8V is outside of the operating voltage specified for imx8mq, see
chapter 3.1.4 "Operating ranges" of the IMX8MDQLQCEC document.

Signed-off-by: Sebastian Krzyszkowiak <sebastian.krzyszkowiak@puri.sm>
Signed-off-by: Martin Kepplinger <martin.kepplinger@puri.sm>
Signed-off-by: Shawn Guo <shawnguo@kernel.org>