arm: socfpga: smc: Add memory coherency support to mailbox command

arm: socfpga: smc: Add memory coherency support to mailbox command

As cache is enabled in U-Boot and disabled in ATF(BL31). We need to
perform cache flush of buffers that are shared between U-Boot and
ATF using secure monitor calls.

Signed-off-by: Mahesh Rao <mahesh.rao@altera.com>
Signed-off-by: Tien Fong Chee <tien.fong.chee@altera.com>
Signed-off-by: Alif Zakuan Yuslaimi <alif.zakuan.yuslaimi@altera.com>
Reviewed-by: Tien Fong Chee <tien.fong.chee@altera.com>