Rework RAM disk driver to be configured using devicetree and
support multiple instances.
This patch also removes a copy of the RAM disk driver,
tests/subsys/fs/fat_fs_dual_drive/src/disk_access_test_drv.c,
that was there for testing multiple disk drivers support.
Bonus: one SYS_INIT() less and a memory region can be exported to the
host.
Signed-off-by: Johann Fischer <johann.fischer@nordicsemi.no>
