Allow DTB to be overridden

Allow DTB to be overridden

Some platforms differ significantly enough from the RTSM VE platform
such the RTSM VE DTB is not a valid DTB for the platform. Now that we
have the appropriate logic for detecting device and memory base
addresses, there's no reason we can't allwo the user to arbittrarily
choose to use another DTB so long as the basics are present (memory, a
pl011, and V2M sysregs).

This patch adds a "--with-dtb" option to override the DTB to use. The
aformentioned base addresses are automatically dicovered at build time.
Where a required device is not present, the build will fail.

Signed-off-by: Mark Rutland <mark.rutland@arm.com>