ARM: dts: msm: keep VDD_MMCX regulator always-on for SDM855 RUMI

Keep the VDD_MMCX regulator supply (pm855l_s5_level) enabled at
all times on SDM855 RUMI.  This is needed in order to avoid
losing the state of retention registers in the VDD_MMCX domain
which are not implemented on SDM855 RUMI.

Change-Id: I988a3d3e249d97dd5dfdec7e97983f0fffa67e38
Signed-off-by: David Collins <collinsd@codeaurora.org>