ARM: dts: mdm: Add stub regulators for mdmfermium

Stub regulators does not perform hardware transactions with the
regulator hardware. Stub regulators allow the regulator consumer
drivers to successfully pass through the regulator callbacks.

Change-Id: I65705fa7b09638c7a371145e3eabcb66cd7709da
Signed-off-by: Ravindranath Thiyagarajan <rthiyaga@codeaurora.org>