reset: reset-mtmips: add DM_FLAG_PRE_RELOC flag

Add DM_FLAG_PRE_RELOC flag for reset-mtmips to make sure this driver can
be probed before relocation even if u-boot,dm-pre-reloc is not present
in the dts.

Reviewed-by: Stefan Roese <sr@denx.de>
Signed-off-by: Weijie Gao <weijie.gao@mediatek.com>