5.4: MIPS: Add Realtek Lexra RLX4181 CPU and RTL8196E SoC support

The Realtek Lexra RLX4181 CPU [1][2] implements the MIPS-I ISA,
without unaligned load/store instructions (lwl, lwr, swl, swr).
The programming model of this CPU is very similar
to the R3000 programming model, with a few differences.

The Realtek RTL8196E SoC is a MIPS based SoC used in some
home routers. The hardware includes RLX4181 CPU with Ethernet
controllers, WLAN controller and more.

This patch includes:
- Adding Kconfig entries for RLX4181
- Adding CPU_RLX4181 to the cpu_type_enum
- Passing -march=rlx4181 to the compiler
- Using existing R3000 code/behavior where possible
- Wait instruction support (for better idle power consumption)
- RDHWR instruction emulation from the page fault handler
- New MIPS rtl8196e platform (mostly DT based)
- defconfig file
- MIPS zboot UART support
- Device tree files for the RTL8196E SoC and Totolink N100RE
router

[1] https://www.realtek.com/en/products/communications-network-ics/item/rtl8196e
[2] https://wikidevi.wi-cat.ru/Realtek/SoC

Signed-off-by: Shibajee Roy <ador250@protonmail.com>