ARM: dts: msm: add LDO 6 fixed regulator for pmxpoorwills

Add a fixed-regulator device for PMXPOORWILLS LDO 6.  This
regulator is physically kept enabled at all times and there is no
mechanism to modify its state.  The fixed-regulator device can be
used by consumers of LDO 6 to satisfy their supply requirements.

Change-Id: Iaa651c8f8f069da10c1de1984de73adf7e41bf82
Signed-off-by: David Collins <collinsd@codeaurora.org>