Flash driver for STM32G0xx and STM32G4xx

Flash driver for STM32G0xx and STM32G4xx

Flash module of STM32G0/G4 family is quite similar to the one of
STM32L4, so only minor changes are required, in particular
adaption of flash loader to Cortex-M0. Register addresses
passed to flash loader to simplify integration of L5.
Added re-probe after option byte load.
Added flash size override via cfg file.
WRPxxR mask now based on max. number of pages instead of fixed 0xFF,
as G4 devices fill up unused bits with '1'.
Sizes in stm32l4_probe changed to multiples of 1kB.

Tested with Nucleo-G071RB, G030J6, Nucleo-G431RB and Nucleo-G474RE.
Gap handling in G4 Cat. 3 dual bank mode tested with STM32G473RB.
This handling isn't optimal as the bank size includes the
size of the gap. WB not tested.

Change-Id: I24df7c065afeb71c11c7e96de4aa9fdb91845593
Signed-off-by: Andreas Bolsch <hyphen0break@gmail.com>
Reviewed-on: http://openocd.zylin.com/4807
Tested-by: jenkins
Reviewed-by: Tarek BOCHKATI <tarek.bouchkati@gmail.com>
Reviewed-by: Tomas Vanek <vanekt@fbl.cz>