docs(stm32mp2): correct STM32MP2 frequencies

STM32MP25xA & STM32MP25xC versions run at 1.2GHz.

Signed-off-by: Nicolas Le Bayon <nicolas.le.bayon@st.com>
Change-Id: I75aea682c8e3fa89e7ac1347bb7f9d02f2086222
1 file changed