ramips: use ralink,nr-gpio instead of ralink,num-gpios

ramips: use ralink,nr-gpio instead of ralink,num-gpios

Since commit c1e7738988f5 ("checks: add gpio binding properties check")
dtc treats any *-gpios and *-gpio property as phandle at least during
checks. The only whitelisted property is nr-gpio.

Use ralink,nr-gpio in favour of ralink,num-gpios to get rid of false
positive warnings.

Signed-off-by: Mathias Kresin <dev@kresin.me>