dm: clk: fix PWR_CR3 register's bit 2 name

Fix bit 2 name of PWR_CR3 register to match with the
last STM32H7 reference manual available here :

http://www.st.com/content/st_com/en/support/resources/


resource-selector.html?querycriteria=productId=SS1951$$
resourceCategory=technical_literature$$resourceType=reference_manual

Update also comment about voltage scaling 1 values

Signed-off-by: Patrice Chotard <patrice.chotard@st.com>
Reviewed-by: Vikas Manocha <vikas.manocha@st.com>