flash/stm32h7x: use alignment infrastructure

flash/stm32h7x: use alignment infrastructure

Report the 32-byte alignemnt requirement via the bank structure rather
than enforcing it ad-hoc in the write routine. This allows people to do
non-32-byte-aligned writes if they want, with the infrastructure fixing
up the addresses passed to the low-level driver.

Change-Id: I2c4f532f2000435954a900224dbc9f2c30d1cc94
Signed-off-by: Christopher Head <chead@zaber.com>
Reviewed-on: http://openocd.zylin.com/5388
Reviewed-by: Tarek BOCHKATI <tarek.bouchkati@gmail.com>
Tested-by: jenkins
Reviewed-by: Tomas Vanek <vanekt@fbl.cz>