ramips: add support for HiWiFi HC5661A

ramips: add support for HiWiFi HC5661A

HC5661A is almost the same as HC5661 but MT7628AN is used instead of MT7620A.

- MT7628AN
- 128 MiB DDR2 RAM (W971GG6KB-25)
- 16 MiB SPI NOR flash (W25Q128)
- SD slot (not work yet)
- 1+4 x 100M Ethernet
- 802.11 b/g/n Wi-Fi
- 3 x LED
- 1 x button
- UART pad on PCB (JP1: TX, RX, GND, 3.3V)

The factory flash layout seems different from HC5661.
"hwf_config" is renamed to "oem" and its size changes to 0x20000.
It is modified accordingly in the dts file.

0x000000000000-0x000000030000 : "u-boot"
0x000000030000-0x000000040000 : "hw_panic"
0x000000040000-0x000000050000 : "Factory"
0x000000050000-0x000000160000 : "kernel"
0x000000160000-0x000000fc0000 : "rootfs"
0x000000bb0000-0x000000fc0000 : "rootfs_data"
0x000000fc0000-0x000000fe0000 : "oem"
0x000000fe0000-0x000000ff0000 : "bdinfo"
0x000000ff0000-0x000001000000 : "backup"
0x000000050000-0x000000fc0000 : "firmware"

To install LEDE, enabled the "developer mode",
which will *void your warranty* and open the SSH server at port 1022.

sysupgrade -n -F lede-ramips-mt7628-hc5661a-squashfs-sysupgrade.bin

SD slot:
- Tried to add modules kmod-sdhci kmod-sdhci-mt7620, and corresponding dts block.
- It will block WAN + 3xLAN ports, only one LAN works.
- I'm not sure why, everything else works fine.

Signed-off-by: Wang JiaWei <buaawjw@gmail.com>