lregion_overlap() is only used by common/startup.c

There is no need to define it in include/common.h

While I was here, I also renamed it to region_overlap(), and got rid
of the 'inline' keyword.

Signed-off-by: Masahiro Yamada <masahiroy@kernel.org>
Signed-off-by: Sascha Hauer <s.hauer@pengutronix.de>