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>

target/linux/ath79/mikrotik/config-default

@@ -2,16 +2,20 @@ CONFIG_AT803X_PHY=y
 CONFIG_CRC16=y
 CONFIG_CRYPTO_DEFLATE=y
 CONFIG_GPIO_LATCH=y
+CONFIG_GPIO_RB4XX=y
 CONFIG_GPIO_WATCHDOG=y
 CONFIG_GPIO_WATCHDOG_ARCH_INITCALL=y
 CONFIG_LEDS_RESET=y
 CONFIG_LZO_DECOMPRESS=y
+CONFIG_MDIO_GPIO=y
+CONFIG_MFD_RB4XX_CPLD=y
 CONFIG_MIKROTIK=y
 CONFIG_MIKROTIK_RB_SYSFS=y
 CONFIG_MTD_NAND=y
 CONFIG_MTD_NAND_AR934X=y
 CONFIG_MTD_NAND_CORE=y
 CONFIG_MTD_NAND_ECC=y
+CONFIG_MTD_NAND_RB4XX=y
 CONFIG_MTD_RAW_NAND=y
 CONFIG_MTD_ROUTERBOOT_PARTS=y
 CONFIG_MTD_SPI_NAND=y
@@ -24,10 +28,12 @@ CONFIG_MTD_UBI_BEB_LIMIT=20
 # CONFIG_MTD_UBI_FASTMAP is not set
 # CONFIG_MTD_UBI_GLUEBI is not set
 CONFIG_NET_SWITCHDEV=y
+CONFIG_PCI_AR71XX=y
 CONFIG_PHY_AR7100_USB=y
 CONFIG_PHY_AR7200_USB=y
 CONFIG_REGULATOR=y
 CONFIG_REGULATOR_FIXED_VOLTAGE=y
+CONFIG_SPI_RB4XX=y
 CONFIG_UBIFS_FS=y
 CONFIG_UBIFS_FS_ADVANCED_COMPR=y
 CONFIG_UBIFS_FS_LZO=y