power: reset: Download mode is not supported in secure boot

Currently msmpower off driver is not checking whether device
is in secure boot or not. So use generic API scm_is_secure_device()
and take appropriate decision to enable download feature.

Change-Id: Ic7bbe1f7c9b839ad5d1d07a6583f6a1b76bf8962
Signed-off-by: Prasad Sodagudi <psodagud@codeaurora.org>