uniphier: revise SCP protocol handshake

When the SoC issues a command IRQ to SCP, SCP sets STMTOBEIRQ as ACK.
The SoC must wait for it before issuing the next command.

This commit makes sure to meet the requirement.

Signed-off-by: Dai Okamura <okamura.dai@socionext.com>
Signed-off-by: Masahiro Yamada <yamada.masahiro@socionext.com>