drivers: flash: it8xxx2: Add missing Kconfigs

The original hwmv1 Kconfigs for this flash driver were in the
complete wrong place and should not have been merged, which then
caused confusion when porting to hwmv2, this adds in the missing
Kconfigs to where they should have been in the first place

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