msm: spm_devices: do not program SPM on cores more than present

As some hardware can have less then 4 cores for a different
version of same target and share the same device tree,
check at runtime for programming SPM only on cores that are
present on the target.

Change-Id: Iad88f902a2ccc0519147ac16819e75ab9c42c14c
Signed-off-by: Priyanka Mathur <pmathur@codeaurora.org>