This commit adds support for the GL-AR750 (2.4G radio only)

This commit adds support for the GL-AR750 (2.4G radio only)

While this router does have an 802.11ac chipset (QCA9887) which
requires non-free firmware (loadable firmware blobs), the main raido in the SoC
can still be used and does not require non-free firmware / blobs since it
is an ath9k chipset...

As it stands, it is not possible to use the 802.11ac radio due to lack of
drivers, the firmware loading mech. has been removed (linux-libre kernel)
and libreCMC does not include or pull needed firmware. The libreCMC project is
not endorsing the usage of the non-free chpset and the barriers are sufficient
that no one can use libreCMC with the non-free components.

TLDR; It is not possible to use the non-free chipset with libreCMC but the device
can still be used as a wireless router without non-free blobs.

Specification:

- 650/597/216 MHz (CPU/DDR/AHB)
- 128 MB of RAM (DDR2)
- 16 MB of FLASH (SPI NOR)
- 3x 10/100 Mbps Ethernet
- 2T2R 2.4 GHz (QCA9531)
- 1T1R 5 GHz (QCA9887)
- 1x USB 2.0 (power controlled by GPIO)
- 1x microSD (GL857L)
- 3x LED (all driven by GPIO)
- 1x button (reset)
- 1x 2-pos switch
- header for optional PoE module
- 1x micro USB for main power input
- UART + I2C header on PCB

Based upon upstream commit : 2e5252d346e2ec832a203af778b5c1d949f0ae5f