ramips: cleanup the RB750Gr3 support

ramips: cleanup the RB750Gr3 support

Always enable the pwr led and use the usr led for boot status indication.

Rename nodes in the dts, to match what is recommend in the devicetree
specification.

Increase the maximum spi frequency to 20MHz and drop the m25p,chunked-io
which isn't required on mt7621.

Use the BTN_0 keycode for the mode button. This board doesn't have any
wireless.

Use a more descriptive label for the reset button and the GPIO enabling
the usb vcc supply.

Use the beeper kernel module for the buzzer.

Fix the pinmux to switch only pins used as GPIOs to the GPIO function.
Add support for the PoE enable GPIO to the userspace. The PoE power
status can be read via GPIO7. Since OpenWrt doesn't have support for
reading inputs from userspace, prepare only the pinmux for the GPIO.

Signed-off-by: Anton Arapov <arapov@gmail.com>
Signed-off-by: Thibaut VARÈNE <hacks@slashdirt.org>
Signed-off-by: Mathias Kresin <dev@kresin.me>