ARM: at91: add code to handle secure calls

ARM: at91: add code to handle secure calls

Since OP-TEE now has a more complete support for sama5d2, add necessary
code to perform SMC calls. The detection of OP-TEE is based on a
specific device-tree node path (/firmware/optee) such has done by some
other SoC. A check is added to avoid doing SMC calls without having
OP-TEE.

Signed-off-by: Clément Léger <[email protected]>
Signed-off-by: Claudiu Beznea <[email protected]>
Signed-off-by: Nicolas Ferre <[email protected]>