drivers: ramdisk: use devicetree to instantiate RAM disk

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>