ARM: at91: add sam_linux_is_optee_available() function

Add sam_linux_is_optee_available() which allows to know if OP-TEE is
available for Linux. This function is used by code which needs to
know if we running with OP-TEE available or not.

Signed-off-by: Clément Léger <clement.leger@bootlin.com>
[claudiu.beznea: edit commit title and message, renamed
 sam_linux_is_in_normal_world() into sam_linux_is_optee_available()]
Signed-off-by: Claudiu Beznea <claudiu.beznea@microchip.com>
Link: https://lore.kernel.org/r/20220606145701.185552-2-clement.leger@bootlin.com