ramips: add support for UniElec U7621-06-512M-64M variant

ramips: add support for UniElec U7621-06-512M-64M variant

Add support for UniElec U7621-06 variant with 512MB RAM and 64MB flash.

Additional specs are below:

CPU: MT7621 (880Mhz)
Bootloader: Ralink U-Boot
Flash: 64MB
 - U-Boot identifies as Macronix MX66L51235F
 - kernel identifies as MX66L51235l (65536 Kbytes)
RAM: 512MB

Rest of the details as per commit 46ab81e405d2 ("ramips add support for
UniElec U7621-06")

Signed-off-by: Nishant Sharma <nishant@unmukti.in>
[use generic board detection, add firmware partition compatible, extend
firmware partition to use all of the remaining flash space, add a
maximum image size matching the firmware partition size]
Signed-off-by: Mathias Kresin <dev@kresin.me>