mailbox: mtk-cmdq: add gce software ddr enable private data

if gce work control by software, we need set software enable
for MT8186 Soc

there is a handshake flow between gce and ddr hardware,
if not set ddr enable flag of gce, ddr will fall into idle
mode, then gce instructions will not process done.
we need set this flag of gce to tell ddr when gce is idle or busy
controlled by software flow.

0x48[2:0] means control by software
0x48[18:16] means ddr enable
0x48[2:0] is pre-condition of 0x48[18:16].
if we want set 0x48[18:16] ddr enable, 0x48[2:0] must be set at same
time.
and only these bits is useful, other bits is useless bits

Signed-off-by: Yongqiang Niu <yongqiang.niu@mediatek.com>
Reviewed-by: CK Hu <ck.hu@mediatek.com>
Signed-off-by: Jassi Brar <jaswinder.singh@linaro.org>