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/rp2040.c

@@ -520,7 +520,7 @@ FLASH_BANK_COMMAND_HANDLER(rp2040_flash_bank_command)
 	return ERROR_OK;
 }
 
-struct flash_driver rp2040_flash = {
+const struct flash_driver rp2040_flash = {
 	.name = "rp2040_flash",
 	.flash_bank_command = rp2040_flash_bank_command,
 	.erase =  rp2040_flash_erase,