refactor(allwinner): move DTB change code into allwinner/common

So far the H616 was the only Allwinner SoC needed to amend the DTB, to
reserve the DRAM portion that BL31 occupies.
To allow other SoCs to modify the DTB as well, without duplicating code,
move the DTB change routines into Allwinner common code, and generalise
the current code to allow other modifications.

No functional change intended.

Change-Id: I080ea07b6470367f3c2573a4368f8ef5196d411c
Signed-off-by: Andre Przywara <andre.przywara@arm.com>
Signed-off-by: Samuel Holland <samuel@sholland.org>
5 files changed