dts: arm: nxp: lpc55S1x: Fix unit and first address mismatch

This fixes the following warning:

> unit address and first address in 'reg' (0x3fc70) don't match for
> /soc/peripheral@40000000/flash-controller@34000/flash@9fc70

Signed-off-by: Reto Schneider <reto.schneider@husqvarnagroup.com>