nor: move in driver.h the flash_driver's declaration

The static analyser 'sparse' complains, while compiling a nor
driver, that the struct flash_driver is declared in the file as
non static, but it is not exposed through an include file.
The message is:
	warning: symbol 'XXX' was not declared. Should it be static?

Move the list of flash_driver's declaration in driver.h
Fix some incorrect non-const declaration and remove redundant
forward declarations.

Change-Id: I5e41d094307aac4a57dfa9a70496ff3cf180bd92
Signed-off-by: Antonio Borneo <borneo.antonio@gmail.com>
Reviewed-on: https://review.openocd.org/c/openocd/+/7662
Tested-by: jenkins

src/flash/nor/at91sam4.c

@@ -62,8 +62,6 @@
 #define  OFFSET_EFC_FSR   8
 #define  OFFSET_EFC_FRR   12
 
-extern const struct flash_driver at91sam4_flash;
-
 static float _tomhz(uint32_t freq_hz)
 {
 	float f;