add support for nordic nrf54lm20

[rjeastwest]

This is support added for the Nordic NRF54LM20, and specifically for the NRF54LM20-DK development board.

[danielinux]

Thanks for the contribution! This port looks well done. I should get a board today or tomorrow to test. Meanwhile after a first look:

• An entry in docs/Targets.md is missing. We usually put "getting started" information about the specific target in that file.
• To ensure we don't break this, we should add at least a build test as github workflow.
• It is unusual to toggle LEDs or in general any interaction with the bootloader is discouraged. I would kindly ask you to move the nrf54lm20_dk.c code to test_app, and feel free to call monitor/tests/led blinking from there at will. The secure bootloader should not include hal_monitor_init, and that should not be part of the hal_ workspace, neither should the bootloader expose any unnecessary interactions except the very devices used for booting/updating (e.g. SPI flash or UART updates). We do have a wolfBoot_printf facility that could be mapped to the UART if you want (not all targets include this) but it should be disabled by default unless PRINTF_ENABLED is explicitly set.
• Avoid initializing grtc and/or calling sleep in the bootloader. Avoid unnecessary interrupts. Keep the bootloader API minimal (hal_init to set clocks and initialize peripherals, flash write/erase driver, hal_prepare_boot to deinitialize peripherals and unset clocks if needed) + optional uart.