v0.2.2 — nRF54LM20A stack top fix

Fixes the nRF54LM20A stack pointer landing in inaccessible RAM.

__StackTop was 0x20080000, the nominal top of the 512 KB. That address is
not usable on silicon, so the first instruction of SystemInit HardFaulted and
the core locked up before reaching main(). The RAM limit is now 0x2007FCC0,
matching where Nordic's own sdk-nrf-bm devicetree ends app_ram.

Verified over SWD on a XIAO nRF54LM20A: the bootloader now reaches
bootloader_dfu_start() and waits for a serial DFU host.

Known limitation: nothing is placed at 0x00000000, so a board flashed with only
this bootloader still locks up at reset. nRF54L has no MBR, and Nordic's model
puts the reset vector owner at 0x0. That is still unresolved.

nRF54L05/L10/L15 are unchanged.