trusty: Retry std_calls on SM_ERR_BUSY

If the trusty spinlock is held, or if the strex fails for another
reason, trusty returns SM_ERR_BUSY. Add retry code to handle this.

Without this retry code, std_calls can fail. If the previous smc
call had returned SM_ERR_INTERRUPTED, this failure would cause
the driver to get out of sync with trusty. All later calls would
then fail with SM_ERR_INTERLEAVED_SMC.

Change-Id: I9f72bb7ce9af9e1ef0a38aeff62fb83ea51377cf
Signed-off-by: Arve Hjønnevåg <arve@android.com>