spm: read number of SPM sequence entries supported

Read number of SPM command sequence entries from SAW2 ID
register for a particular SPM. On this basis limit the
number of command writes.

Change-Id: I440442b1ab3fa8f13cc9b095b443aa77a9c6a702
Signed-off-by: Anil kumar mamidala <amami@codeaurora.org>