make: Add a command line option for debugging OpenSBI

Add a new make command line option "make DEBUG=1" to prevent compiler
optimizations using -O2.

Signed-off-by: Bin Meng <bmeng@tinylab.org>
Reviewed-by: Xiang W <wxjstz@126.com>
Reviewed-by: Anup Patel <anup@brainfault.org>