invoke_smc() allow U-Boot proper running in non-secure mode (EL2)
to invoke SMC call to ATF's PSCI runtime services such as
System Manager's registers access, 2nd phase bitstream FPGA
reconfiguration, Remote System Update (RSU) and etc.

smc_send_mailbox() is a send mailbox command helper function which invokes
the ATF's PSCI runtime service (function ID: INTEL_SIP_SMC_MBOX_SEND_CMD)
to send mailbox messages to Secure Device Manager (SDM).
Signed-off-by: NChee Hong Ang <chee.hong.ang@intel.com>