Makefile: Add cscope support

Add cscope support so that running `make cscope` will generate/update
cscope files used for source code browsing, while running `make
distclean` will remove the cscope files.

Also add entry in .gitignore to ignore generated cscope files.

Signed-off-by: Tan En De <ende.tan@linux.starfivetech.com>
Reviewed-by: Xiang W <wxjstz@126.com>
Reviewed-by: Anup Patel <anup@brainfault.org>
Reviewed-by: Andrew Jones <ajones@ventanamicro.com>