dts: common: nordic: nrf52840_partitions: Use equal partitions

Switches back to equal sized partitions, this fixes an issue
whereby the number of overhead sectors for a swap mode was
incorrectly listed as 2 when it should have been 1, and also
allows using any swap mode. This means that when using swap
using mode, 1 sector in the secondary partition will be unusable,
and when using swap using offset, 1 sector in the primary
partition will be unusable

Signed-off-by: Jamie McCrae <jamie.mccrae@nordicsemi.no>