MAINTAINERS: Add STM32MP1 RNG driver in stm32mp platform

Add the STM32MP1 RNG driver in the list of drivers supported by
the STMicroelectronics STM32MP15x series.

Signed-off-by: Patrick Delaunay <patrick.delaunay@st.com>
Reviewed-by: Patrice Chotard <patrice.chotard@st.com>