ARM: dts: qcom: add VDD_MX and CX subnodes for MMCX dependencies on Kona

Add VDD_MX and VDD_CX rpmh-regulator subnodes which are used by
the VDD_MMCX regulator device.  Configure these sub nodes so that
a request for VDD_CX = minimum operating level is present any
time that VDD_MMCX is enabled.

This ensures that multimedia hardware blocks are not accidentally
reset due to VDD_MMCX being enabled when VDD_CX is not enabled.

Change-Id: I96156b47b941a6fed92516388556cbdd196905fe