qseecom: add configurable encryption key size

ICE crypto engine key size can now be configured using dtsi flag.
This is required to facilitate usage of common TZ image with variable
key sizes and also provides flexibility in handling different key sizes
requirement in future. This flag is parsed in driver and depending upon
the value HLOS decides on key size to be passed to TZ.

Change-Id: If40f83a943e5ae8333d171c21823a16816d7ddd4
Signed-off-by: Neeraj Soni <neersoni@codeaurora.org>
Signed-off-by: Andrey Markovytch <andreym@codeaurora.org>