drivers/flash/Ambiq: Set erase capabilities, depending on SoC

The commit selects HAS_NO_EXPLICIT_ERASE for Apollo4,
with MRAM, and HAS_EXPLICIT_ERASE for Apollo3 which has Flash,
for a driver that shares code between SoCs.

In case of Apollo4 the no_explicit_erase capability is set to
true, while it remains false for Apollo3.

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