boards/arm: unify boot partition size for nRF boards

The DTS for nRF53 and nRF91 boards has been modified to cut the size
of boot partition from 64KiB to 48KiB, making it the same size as for
nRF52.

Resolves #25664

Signed-off-by: Dominik Ermel <dominik.ermel@nordicsemi.no>