A lot of the features in here are only relevant when running
multi-threaded with interrupts. Drop everything except what we need to
run single-threaded with a single TCS (which is all the rpmh-rsc
framework in U-Boot supports).
Keep rpmh_write_async() for simplicity and make it wrap the regular
rpmh_write().
Acked-by: Sumit Garg <sumit.garg@linaro.org>
Signed-off-by: Caleb Connolly <caleb.connolly@linaro.org>