soc: ambiq: apollo3x: Flash-controller reconfigured for mcuboot

Due to Apollo3's internal bootloader, zephyr build is not able
to create correct flash address on linker.cmd while using
mcuboot. The PR configures flash-controller start address
to solve this problem.

Test board: rakwireless/rak11720
Test project: samples/subsys/mgmt/mcumgr/smp_svr

Signed-off-by: Sercan Erat <sercanerat@gmail.com>