fix(st-crypto): do not read RNG data if it's not ready

Having RNG_SR_DRDY bit in RNG_SR register does not mean that there are 4
RNG words ready to be read. Add a check on RNG_SR_DRDY between each word
reading.

Signed-off-by: Gatien Chevallier <gatien.chevallier@foss.st.com>
Change-Id: I46af7ca6c0ddbe19540b248365a5016b15b9a707
Reviewed-on: https://gerrit.st.com/c/mpu/oe/st/tf-a/+/308887
Tested-by: Gatien CHEVALLIER <gatien.chevallier@st.com>
Reviewed-by: Gatien CHEVALLIER <gatien.chevallier@st.com>
Reviewed-by: Yann GAUTIER <yann.gautier@foss.st.com>
ACI: CITOOLS <MDG-smet-aci-reviews@list.st.com>
ACI: CIBUILD <MDG-smet-aci-builds@list.st.com>
Domain-Review: Lionel DEBIEVE <lionel.debieve@foss.st.com>

Author: GseoC

drivers/st/crypto/stm32_rng.c

@@ -187,6 +187,10 @@ int stm32_rng_read(uint8_t *out, uint32_t size)
 
 		count = 4U;
 		while (len != 0U) {
+			if ((mmio_read_32(stm32_rng.base + RNG_SR) & RNG_SR_DRDY) == 0U) {
+				break;
+			}
+
 			data32 = mmio_read_32(stm32_rng.base + RNG_DR);
 			count--;