flash/nor: move variable's declaration in C file

flash/nor: move variable's declaration in C file

Variables should not be declared in the include file, otherwise
multiple include will cause multiple instances.

Move the declaration in the C file and make it static.

Change-Id: I8b4884cba643a792a78df4e123aa324b19d92279
Signed-off-by: Antonio Borneo <borneo.antonio@gmail.com>
Reviewed-on: https://review.openocd.org/c/openocd/+/7172
Reviewed-by: Tomas Vanek <vanekt@fbl.cz>
Tested-by: jenkins