flash/stm32l4x: always use stm32l4_get_flash_reg

flash/stm32l4x: always use stm32l4_get_flash_reg

this change is a preparation for STM32L5 support on top of L4 driver
STM32L5 flash is quite similar to L4 flash, mainly register names
and offsets and some bits are changed.
a table with register offset will be introduced, thus correct register
addresses will be obtained using this table and the driver internal
function 'stm32l4_get_flash_reg' will be responsible of this.

Change-Id: I74bf61a83fe53575623640af0328b3253ecc796f
Signed-off-by: Tarek BOCHKATI <tarek.bouchkati@gmail.com>
Reviewed-on: http://openocd.zylin.com/5508
Tested-by: jenkins
Reviewed-by: Andreas Bolsch <hyphen0break@gmail.com>
Reviewed-by: Michael Jung <mijung@gmx.net>
Reviewed-by: Tomas Vanek <vanekt@fbl.cz>