ath79: generic: rework ar9342_ubnt_xw dtsi, and add support for Ubiquiti LiteBeam...

ath79: generic: rework ar9342_ubnt_xw dtsi, and add support for Ubiquiti LiteBeam M5 (XW), Ubiquiti AirGrid M5 HP (XW), Ubiquiti PowerBeam M5 300 (XW)

Add support for Ubiquiti LiteBeam M5 (XW).
The device was previously supported in ar71xx.
See commit: https://git.openwrt.org/?p=openwrt/openwrt.git;a=commit;h=d0988235dd277b9a832bbc4b2a100ac6e821f577

Add ALTX_MODEL for Ubiquiti AirGrid M5 HP (XW), Ubiquiti PowerBeam M5 300 (XW) in generic-ubnt.mk
This models are identical (firmware-wise) to the already supported Ubiquiti Nanostation Loco M (XW)

Add also Ubiquiti NanoBeam M5 to ALTX_MODEL of Ubiquiti Nanostation Loco M (XW) since it's another clone.

Tested on:
- Ubiquiti LiteBeam M5 (XW)
- Ubiquiti PowerBeam M5 (XW)

This also modify target/ath79/dts/ar9342_ubnt_xw.dtsi to use nvmem for calibration data
Checked that the caldata size in the eeprom partition are actually 0x440 on:
- Ubiquiti PowerBeam M5 (XW)
- Ubiquiti Nanostation M5 (XW)
- Ubiquiti LiteBeam M5 (XW)
- Ubiquiti AirGrid M5 HP (XW)

Signed-off-by: Samuele Longhi <agave@dracaena.it>