mediatek: update openembed som7981 support

mediatek: update openembed som7981 support

The board has been redesigned due to previous hardware bugs
(with other reasons maybe).

Changes in new board:
- Added a gpio beeper
- Added a Atmel i2c eeprom
- Added a Atmel i2c ECC accelerator
- Added a Philips RTC module
- Added two RS485
- Removed WPS button
- Replaced USB3 port with M.2 B-key for LTE modules
- Swapped GbE LEDs gpio

Also assigned wifi mac with nvmem binding, added iface setup for failsafe,
increased phy assert time for rtl8221b, and updated LED labels.

Keeping compatibility for old version is not necessary here as only
few samples were sent to those interested in it.

Signed-off-by: Tianling Shen <cnsztl@immortalwrt.org>
Link: https://github.com/openwrt/openwrt/pull/17253
Signed-off-by: Hauke Mehrtens <hauke@hauke-m.de>