x86: Separate early_res related code from e820.c

... to make e820.c smaller.

-v2: fix 32bit compiling with MAX_DMA32_PFN

Signed-off-by: Yinghai Lu <yinghai@kernel.org>
LKML-Reference: <1265793639-15071-21-git-send-email-yinghai@kernel.org>
Signed-off-by: H. Peter Anvin <hpa@zytor.com>