ARM: stm32mp: Add STM32MP21 support

ARM: stm32mp: Add STM32MP21 support

STM32MP21 application processors (STM32 MPUs) based on a single
Arm Cortex®-A35 core running up to 1.5 GHz and Cortex®-M33 core
running at 300 MHz.

It is pin-compatible with the STM32MP2 series in the VFBGA361
10×10 mm package: the STM32MP21 uses a subset of the STM32MP23
pinout, which itself is a subset of the STM32MP25.

More details available here :
https://www.st.com/en/microcontrollers-microprocessors/stm32mp2-series.html

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