ARM: dts: msm: add pmxprairie VREF RGMII regulator device for sdxprairie

Add an rpmh-regulator device for the VREF_RGMII peripheral found
in pmxprairie PMIC chips.  This is needed for EMAC use cases.

Change-Id: I28cd8d82b4af654e2742df153725eec44964244c
Signed-off-by: David Collins <collinsd@codeaurora.org>