review fixes

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Before this patch: The normal world was only allowed to enter
secure world with one thread at a time.

After this patch: The normal world may try to enter secure world
with as many threads as it likes, secure world will return busy
when no more threads can be allowed. Secure world still only allows
one active thread at a time, but during RPC another thread may enter
and do some work. This is needed for cancellation to work.

This patch depends on the "Allow parallel entries to secure world"
patch in optee_linuxdriver.
mutex: bugfix
Reentrancy fixes 2