icnss: set vdd-0.8-cx-mx regulator optional for icnss driver

The wlan firmware is configuring the clk and voltage regulator
for hardware power on and hardware reset sequence.

The icnss platform driver is configuring this clk and regulators
to avoid invalid access to copy engine register during the wlan
recovery.

As platform driver is not configuring this regulator to power on
or reset the WLAN hardware, So no need to define the voltage
regulators are the required properties for the ICNSS platform driver.

Define vdd-0.8-cx-mx as an optional voltage regulator for
the icnss platform driver.

CRs-Fixed: 2014965
Change-Id: I74a1445522997ba4e7a52da5a29e9afb57ad12df
Signed-off-by: Sarada Prasanna Garnayak <sgarna@codeaurora.org>