rng: add OP-TEE based Random Number Generator

Add driver for OP-TEE based Random Number Generator on ARM SoCs
where hardware entropy sources are not accessible to normal world
and the RNG service is provided by a HWRNG Trusted Application (TA).

This driver is based on the linux driver: char/hw_random/optee-rng.c

Signed-off-by: Patrick Delaunay <patrick.delaunay@foss.st.com>
diff --git a/MAINTAINERS b/MAINTAINERS
index 0fc034f..6142504 100644
--- a/MAINTAINERS
+++ b/MAINTAINERS
@@ -486,6 +486,7 @@
 F:	drivers/ram/stm32mp1/
 F:	drivers/remoteproc/stm32_copro.c
 F:	drivers/reset/stm32-reset.c
+F:	drivers/rng/optee_rng.c
 F:	drivers/rng/stm32mp1_rng.c
 F:	drivers/rtc/stm32_rtc.c
 F:	drivers/serial/serial_stm32.*