docs: stm32mp1: add links to documentation

A link to st.com page describing STM32MP1 is added.
Add the information about Cortex-M4 embedded in STM32MP1.
Correct typo for u-boot command.

Change-Id: Ie900f6ee59461c5e7ad8a8b06854abaf41fca3ce
Signed-off-by: Yann Gautier <yann.gautier@st.com>
1 file changed