airoha: Add new target platform

Airoha is a new ARM platform based on Cortex-A53 which has recently been
merged into linux-next.

Due to BootROM limitations on this platform, the Cortex-A53 can't run in
Aarch64 mode and code must be compiled for 32-Bit ARM.

This support is based mostly on those linux-next commits backported
for kernel 5.15.

Patches:
1 - platform support = linux-next
2 - clock driver = linux-next
3 - gpio driver = linux-next
4 - linux,usable-memory-range dts support = linux-next
5 - mtd spinand driver
6 - spi driver
7 - pci driver (kconfig only, uses mediatek PCI) = linux-next

Still missing:
- Ethernet driver
- Sysupgrade support

A.t.m there exists one subtarget EN7523 with only one evaluation
board.

The initramfs can be run with the following commands from u-boot:
-
u-boot> setenv bootfile \
	openwrt-airoha-airoha_en7523-evb-initramfs-kernel.bin
u-boot> tftpboot
u-boot> bootm 0x81800000
-

Signed-off-by: Daniel Danzberger <daniel@dd-wrt.com>

target/linux/airoha/Makefile

@@ -0,0 +1,15 @@
+include $(TOPDIR)/rules.mk
+
+ARCH:=arm
+BOARD:=airoha
+BOARDNAME:=Airoha ARM
+CPU_TYPE:=cortex-a7
+FEATURES:=dt squashfs nand ramdisk gpio source-only
+
+KERNEL_PATCHVER:=5.15
+
+include $(INCLUDE_DIR)/target.mk
+
+KERNELNAME:=Image dtbs
+
+$(eval $(call BuildTarget))