ipq40xx: Linksys MR8300: fix the USB port power

ipq40xx: Linksys MR8300: fix the USB port power

The USB port on the MR8300 randomly fails to feed bus-powered devices.

This is caused by a misconfigured pinmux. The GPIO68 should be used to
enable the USB power (active low), but it's inside the NAND pinmux.

This GPIO pin was found in the original firmware at a startup script in
both MR8300 and EA8300. Therefore apply the fix for both boards.

Signed-off-by: Daniel González Cabanelas <dgcbueu@gmail.com>
Reviewed-by: Robert Marko <robimarko@gmail.com>