meta-ampere: mtjade: fix PCIE AER when Host shutdowns

Switch OCP power source from 3V3 to standby too early causes PCIE AER
error on the OCP adapter.
This commit changes to switch the OCP power source via OCP_MAIN_PWREN in
power control application to avoid the issue.

Tested:
1. Run IPMI chassis power soft command and check if no PCIE AER error
generated.

Signed-off-by: Thang Q. Nguyen <thang@os.amperecomputing.com>
Change-Id: I9dbc621390fb04fa818a04aeaac53b6b729e9e75