econet: add EN7528 subtarget support

econet: add EN7528 subtarget support

The EN7528 is a little endian dual-core MIPS 1004Kc SoC used in xPON
devices. Unlike the big endian EN751221, EN7528 uses the MIPS GIC
interrupt controller for SMP.

This adds minimal boot support for EN7528:
- New en7528 subtarget with mipsel architecture
- Kernel patches for EN7528 SoC with GIC support
- Timer driver extended to support GIC shared interrupts per CPU
- SPI driver fix for EN7528 chip select handling
- Generic device tree for initial bring-up

Signed-off-by: Ahmed Naseef <naseefkm@gmail.com>
Link: https://github.com/openwrt/openwrt/pull/21326
Signed-off-by: Hauke Mehrtens <hauke@hauke-m.de>