COMMON: Add __stringify() function

Copied from Linux kernel:
commit 8f7c2c37319a81ef4c2bfdec67b1ccd5744d97e4
Date:   Wed Apr 8 16:58:57 2009 +0800

Pull in the __stringify() macro from Linux kernel. This macro is usually used to
convert numbers to strings at preprocessor level, yet it is not limited only to
that. This is useful as it allows higher usage of puts() in favour of printf().

Signed-off-by: Marek Vasut <marex@denx.de>
Cc: Wolfgang Denk <wd@denx.de>