xen/arm: optee: document OPTEE option in tee/Kconfig

Add basic information about the OP-TEE mediator and note about
dependency on virtualization-aware OP-TEE.

Signed-off-by: Volodymyr Babchuk <volodymyr_babchuk@epam.com>
Acked-by: Julien Grall <julien.grall@arm.com>

xen/arch/arm/tee/Kconfig

@@ -2,3 +2,8 @@ config OPTEE
 	bool "Enable OP-TEE mediator"
 	default n
 	depends on TEE
+	help
+	  Enable experimental OP-TEE mediator. It allows guests to access
+	  OP-TEE running on your platform. This requires virtualization-enabled
+	  OP-TEE present. You can learn more about virtualization for OP-TEE
+	  at https://optee.readthedocs.io/architecture/virtualization.html