arm: socfpga: spl: Notify SDM on FSBL execution

arm: socfpga: spl: Notify SDM on FSBL execution

Send out "HPS_STAGE_NOTIFY" mailbox command to the
Secure Device Manager (SDM) in SPL to inform SDM on
FSBL execution.

This is necessary for the SDM to recognize that the
FSBL stage has begun its execution and should be
made as early as possible in the FSBL process.

Therefore, the mailbox will initialize and send out
the notification right after the completion of timer
initialization.

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