plat/arm/sgi: disable Ares cpu power down bit in reset handler

plat/arm/sgi: disable Ares cpu power down bit in reset handler

On SGI platforms that include Ares CPUs, the 'CORE_PWRDN_EN' bit of
'CPUPWRCTLR_EL1' register requires an explicit write to clear it to
enable hotplug and idle to function correctly.

The reset value of the CORE_PWRDN_EN bit is zero but it still requires
this explicit clear to zero. This indicates that this could be a model
related issue but for now this issue can be fixed be clearing the
CORE_PWRDN_EN bit in the platform specific reset handler function.

Change-Id: I8b9884ae27a2986d789bfec2e9ae792ef930944e
Signed-off-by: Chandni Cherukuri <chandni.cherukuri@arm.com>