plat: marvell: Update SUBVERSION to match Marvell's forked version

Marvell's TF-A fork has SUBVERSION set to devel-18.12.2.

The only differences between Marvell's devel-18.12.0 and devel-18.12.2
versions are documentation updates and cherry-picked patches from TF-A
upstream repository.

So upstream TF-A has already all changes from Marvell's TF-A devel-18.12.2
fork and therefore update SUBVERSION to reflect this state.
Signed-off-by: Pali Rohár <pali@kernel.org>
Change-Id: I5ce946a5176a5cbf124acd8037392463d586b072

plat/marvell/version.mk

-SUBVERSION = devel-18.12.0
+SUBVERSION = devel-18.12.2