ar71xx: add support for RB mAP L-2nD

This patch adds support for the MikroTik RouterBOARD mAP lite
https://routerboard.com/RBmAPL-2nD

Specifications:
- SoC: Qualcomm QCA9533 (650MHz)
- RAM: 64MB
- Storage: 16MB NOR SPI flash
- Wireless: builtin QCA9533, 2x2:2
- Ethernet: 1x100M

This is another 16M SPI NOR mikrotik device. The machine file is named
mach-rbspi.c because I plan to add support for several of the other spi-based
RouterBOARD devices in subsequent patches: they share most of the hardware
and thus the same codebase.

Signed-off-by: Thibaut VARENE <hacks@slashdirt.org>