boards: nordic: nrf54l15pdk: fix flashing for `CPUFLPR_XIP`

`board.cmake` specified runner arguments only for the RAM-based variant,
use SOC config to support both:
`CONFIG_BOARD_NRF54L15PDK_NRF54L15_CPUFLPR` and
`CONFIG_BOARD_NRF54L15PDK_NRF54L15_CPUFLPR_XIP`.
Also align ARM target so it looks nice.

Signed-off-by: Marcin Szymczyk <marcin.szymczyk@nordicsemi.no>