ath79: add support for MikroTik RouterBOARD 493G (rb4xx series)

ath79: add support for MikroTik RouterBOARD 493G (rb4xx series)

This patch adds support for the MikroTik RouterBOARD RB493G, ported
from the ar71xx target.

See https://routerboard.com/RB493G for details

Specification:
- SoC Qualcomm Atheros AR7161
- RAM: 256 MiB
- Storage: 128MiB NAND
- Ethernet: 9x 1000/100/10 Mbps
- USB 1x 2.0 / 1.0 type A
- PCIe: 3x Mini slot
- MicroSD slot

Working:
- Board/system detection
- Ethernet
- SPI
- NAND
- LEDs
- USB
- Sysupgrade

Enabled (but untested due to lack of hardware):
- PCIe - ath79_pci_irq struct has the slot/pin/IRQ mappings if needed

Installation methods:
- tftp boot initramfs image, scp then flash via "sysupgrade -n"
- nand boot existing OpenWrt, scp then flash via "sysupgrade -n"

Notes:
- initramfs image will not work if uncompressed image size over ~8.5Mb
- The "rb4xx" drivers have been enabled

Signed-off-by: Christopher Hill <ch6574@gmail.com>