physmem: Move the ram0 device earlier in the boot

If the ram driver is unable to reserve a memory range it will panic
with no real information why. Move this driver earlier in the boot so
any devices that cause the conflict will fail to attach.

This should make it easier to debug why the conflict exists.

Reviewed by: mhorne, kevans, imp
Sponsored by: Arm Ltd
Differential Revision: https://reviews.freebsd.org/D48766