flash/stm32h7x: Rename functions and variable names

Prepare support for STM32H7R/S
Rename methods to follow the STM32l4 driver naming

Change-Id: Iad14ba89a48a63c158dae05a53dcbf92f6fe2f53
Signed-off-by: HAOUES Ahmed <[email protected]>
Reviewed-on: https://review.openocd.org/c/openocd/+/8889
Tested-by: jenkins
Reviewed-by: Tomas Vanek <[email protected]>

Author: ahmed-haoues