arm: mvebu: a37xx: Add support for writing Security OTP values

Implement write support for Security OTP values via mailbox API commands
MBOX_CMD_OTP_WRITE_32B and MBOX_CMD_OTP_WRITE.

Write support for North and South Bridge OTPs are not implemented as these
OTPs are already burned in factory with some data.

Signed-off-by: Pali Rohár <pali@kernel.org>
Reviewed-by: Marek Behún <marek.behun@nic.cz>