ARM: dts: mdm: Use the correct key for MX floor-voting on 9x07

The MX floor regulator incorrectly uses the 'vlvl' key
instead of 'vfl'. This  prevents MX voltage from lowering
down when APPS is in sleep. Fix it.

CRs-Fixed: 1047978
Change-Id: I0731714fcfacb027e8a1dc457db2ca1d0eb1dbf3
Signed-off-by: Anirudh Ghayal <aghayal@codeaurora.org>