input: specify the default of I8042_KEYB in more correct manner

Creating multiple entries of "config FOO" often gives us bad
experiences.  In this case, we should specify "default X86"
as platforms that want this keyboard by default.

Signed-off-by: Masahiro Yamada <yamada.masahiro@socionext.com>
Acked-by: Marek Vasut <marex@denx.de>
Reviewed-by: Simon Glass <sjg@chromium.org>