boards: st: correct RAM and/or Flash size in YAML board description

Fix several RAM size that are wrong regarding what is defined in the
related board device tree.

In many cases, SoC DTSI files have been changed to split internal RAMs
and related boards YAML file was not updated accordingly. This is the
case of H5xx, H7xx

In the case of stm32g081b_eval, polarity bytes are not default enabled
hence the internal SRAM size of 36kB, not 32kB.

Stm32mp135f was wrongly set with 256KB RAM and Flash whereas the board
assigns 256MB for each.

Stm32mp157c_dk2 assigned SRAM1, SRAM2 and SRAM3 to Zephyr hence
providing 320kB of system RAM.

Fix several typos setting RAM size to 786 whereas 768 is expected.

Signed-off-by: Etienne Carriere <etienne.carriere@st.com>