ath79: add support for GL.iNet GL-AR750S

ath79: add support for GL.iNet GL-AR750S

Right now this patch adds nor image generation only. NAND image
generation is not supportet at the moment.

Furtheremore support for the MicroSD port is not implemented as of now.

Specification:
- SOC: QCA9563 (775MHz)
- Flash: 16 MiB (W25Q128FVSG)
- RAM: 128 MiB DDR2
- Ethernet: 2x 1Gbps LAN + 1x 1Gbps WAN
- Wireless: 2.4GHz (bgn) and 5GHz (ac)
- USB: 1x USB 2.0 port
- Button: 1x switch button, 1x reset button
- LED: 3x LEDS (green)
  - Another LED can be accessed on GPIO 7 if soldered

Flash instruction:
- Set static ip to 192.168.1.2
- Unplug the power cord
- Hold reset button
- Plug power back in
- Right led will flash 5 times
- Release reset button
- Browse to 192.168.1.1
- Choose sysupgrade image in NOR-flash section
- Press "update nor firmware"
- After successful transfer unplug network cable before device restarts

Signed-off-by: Christoph Krapp <achterin@googlemail.com>
[resolve merge conflicts, rename buttons, use switch input type for mode
switch]
Signed-off-by: Mathias Kresin <dev@kresin.me>