refactor(cm): introduce a real manage_extensions_nonsecure()

manage_extensions_nonsecure() is problematic because it updates both
context and in-place registers (unlike its secure/realm counterparts).
The in-place register updates make it particularly tricky, as those
never change for the lifetime of TF-A. However, they are only set when
exiting to NS world. As such, all of TF-A's execution before that
operates under a different context. This is inconsistent and could cause
problems.

This patch Introduce a real manage_extensions_nonsecure() which only
operates on the context structure. It also introduces a
cm_manage_extensions_el3() which only operates on register in-place that
are not context switched. It is called in BL31's entrypoints so that all
of TF-A executes with the same environment once all features have been
converted.

Signed-off-by: Boyan Karatotev <boyan.karatotev@arm.com>
Change-Id: Ic579f86c41026d2054863ef44893e0ba4c591da9