Revert "mtd: make mtd_partition.name const"

This reverts commit 26a6d240 ("mtd: make mtd_partition.name const"),
to fix build issues related to mtd_partition.name.

Change-Id: I65516beaded8a148ad2e7fffaea186caa852cc39
Signed-off-by: Pavan Anamula <pavana@codeaurora.org>