ARM: dts: msm: specify a flag to control encryption key size on msm8998

This is required to control the FDE/FBE key size from kernel.
Same TZ image can now be used for different encryption key sizes.
The key size will not be fixed and will be controlled with
this flag in qseecom driver.

Change-Id: Iac5c25ba5dc7d22cd7a4961dd5058db0088995ca
Signed-off-by: Andrey Markovytch <andreym@codeaurora.org>