x86: move arch-specific asmlinkage to <asm/linkage.h>

Commit 65dd74a6 (x86: ivybridge: Implement SDRAM init) introduced
x86-specific asmlinkage into arch/x86/include/asm/config.h.

Commit ed0a2fbf (x86: Add a definition of asmlinkage) added the
same macro define again, this time, into include/common.h.
(Please do not add arch-specific stuff to include/common.h any more;
it is already too cluttered.)

The generic asmlinkage is defined in <linux/linkage.h>.  If you want
to override it with an arch-specific one, the best way is to add it
to <asm/linkage.h> like Linux Kernel.

Signed-off-by: Masahiro Yamada <yamada.m@jp.panasonic.com>
Cc: Simon Glass <sjg@chromium.org>
Tested-by: Simon Glass <sjg@chromium.org>