armv7R: K3: am654: Shut down R5 core after ATF startup on A53

Rather than simply parking the R5 core in WFE after starting up ATF
on A53 instead use SYSFW API to properly shut down the R5 CPU cores
as well as associated timer resources that were pre-allocated. This
allows software further downstream to properly and gracefully bring
the R5 cores back online if desired.

Signed-off-by: Andreas Dannenberg <dannenberg@ti.com>
Signed-off-by: Lokesh Vutla <lokeshvutla@ti.com>