ath79: add support for ELECOM WRC-1750GHBK2-I/C

ath79: add support for ELECOM WRC-1750GHBK2-I/C

ELECOM WRC-1750GHBK2-I/WRC-1750GHBK2-C are 2.4/5 GHz band 11ac
routers, based on Qualcomm Atheros QCA9563.

This commit also includes small fix; use "0x0x03000101" as pll_1000
instead of "0x03000000".

Specification:

- SoC:      Qualcom Atheros QCA9563
- RAM:      128 MB (DDR2)
- Flash:    16 MB (SPI-NOR)
- WLAN:     2.4/5 GHz
  - 2.4 GHz: 2T2R (SoC internal)
  - 5 GHz:   3T3R (QCA9880)
- Ethernet: 10/100/1000 Mbps
- LED/key:  4x/3x (2x buttons, 1x slide-switch)
- UART:     through-hole on PCB
  - Vcc, RX, GND, TX from switch (QCA8337N) side
  - 115200n8

Flash instruction using factory image:

1. Boot WRC-1750GHBK2-I/C normaly
2. Access to "http://192.168.2.1/" and open firmware upgrade page
("ファームウェア更新 手動更新（アップデート）")
3. Select the OpenWrt factory image and click apply ("適用") button
to perform firmware update
4. On the (initramfs) factory image, perform sysupgrade with
squashfs-sysupgrade image
5. Wait ~150 seconds to complete flashing

Signed-off-by: INAGAKI Hiroshi <musashino.open@gmail.com>