x86: Set CONFIG_NR_DRAM_BANKS to 8 and move it to x86-common.h

Some x86 boards set CONFIG_NR_DRAM_BANKS to 1, which causes incorrect
DRAM size printed when booting from coreboot, like this:

	CPU: x86, vendor Intel, device 663h
	DRAM:  636 KiB
	Using default environment

Change it to 8 which should be enough for both coreboot and bare
cases, and move it to x86-common.h.

Signed-off-by: Bin Meng <bmeng.cn@gmail.com>
Acked-by: Simon Glass <sjg@chromium.org>