soc: cpu_pwr_ctl: Add support to configure cpu retention ldo

Add support to configure the cpu retention ldo voltage. This value needs to
be configured when a core is coldbooted. The core will then be able to
enter retention at the programmed retention voltage.

Change-Id: I48e0f19baaf62522cee328437770d601d4f0dfc5
Signed-off-by: Karthik Parsha <kparsha@codeaurora.org>