x86: Move back find_e820_area to e820.c

Makes early_res.c more clean, so later could move it to /kernel.

Signed-off-by: Yinghai Lu <yinghai@kernel.org>
LKML-Reference: <1265793639-15071-23-git-send-email-yinghai@kernel.org>
Signed-off-by: H. Peter Anvin <hpa@zytor.com>