spl: opensbi: wait for ack from secondary harts before entering OpenSBI

At the start, OpenSBI relocates itself to its link address. If the link
address ranges of U-Boot SPL and OpenSBI overlap, the relocation can
lead to code corruption if a hart is still running U-Boot SPL during
relocation. To avoid this problem, the main hart is specified as the
preferred boot hart to perform the relocation. This fixes the code
corruption problems based on the assumption that since the main hart
schedules the secondary harts to enter OpenSBI, it will be the last to
enter OpenSBI. However it was reported that this assumption is not
always correct.

To make sure the assumption always holds true, wait for all secondary
harts to acknowledge the call-function request before entering OpenSBI
on the main hart.

Reported-by: Rick Chen <rick@andestech.com>
Signed-off-by: Lukas Auer <lukas.auer@aisec.fraunhofer.de>
Reviewed-by: Rick Chen <rick@andestech.com>
Tested-by: Rick Chen <rick@andestech.com>
Reviewed-by: Anup Patel <anup.patel@wdc.com>