arm64: dts: librem5: Limit the USB to 5V

The charge controller can handle 14V but the PTC on the devkit can only
handle 6V so limit the negotiated voltage to 5V.

Signed-off-by: Angus Ainslie (Purism) <angus@akkea.ca>
Signed-off-by: Shawn Guo <shawnguo@kernel.org>