ramips: Add support for SERCOMM NA502

The SERCOMM NA502 is a smart home gateway manufactured by SERCOMM and sold
under different brands (among others, A1 Telekom Austria SmartHome
Gateway). It has multi-protocol radio support in addition to LAN and WiFi.

Note: BLE is currently unsupported.

Specifications
--------------

  - MT7621ST 880MHz, Single-Core, Dual-Thread
  - MT7603EN 2.4GHz WiFi
  - MT7662EN 5GHz WiFi + BLE
  - 128MiB NAND
  - 256MiB DDR3 RAM
  - SD3503 ZWave Controller
  - EM357 Zigbee Coordinator

MAC address assignment
----------------------

LAN MAC is read from the config partition, WiFi 2.4GHz is LAN+2 and matches
the OEM firmware. WiFi 5GHz with LAN+1 is an educated guess since the
OEM firmware does not enable 5GHz WiFi.

Installation
------------
Attach serial console, then boot the initramfs image via TFTP.
Once inside OpenWrt, run sysupgrade -n with the sysupgrade file.

Attention: The device has a dual-firmware design. We overwrite kernel2,
since kernel1 contains an automatic recovery image.

If you get NAND ECC errors and are stuck with bad eraseblocks, try to
erase the mtd partition first with

mtd unlock ubi
mtd erase ubi

This should only be needed once.

Signed-off-by: Andreas Böhler <dev@aboehler.at>
[use kiB for IMAGE_SIZE]
Signed-off-by: Adrian Schmutzler <freifunk@adrianschmutzler.de>