input: Provide a board specific mechanism to decide whether to skip i8042 init

This change adds a board overridable function which can be used to decide
whether or not to initialize the i8042 keyboard controller. On systems where
it isn't actually connected to anything, this can save a significant amount of
boot time.

On Stumpy, this saves about 200ms on boot.

Signed-off-by: Gabe Black <gabeblack@chromium.org>
Signed-off-by: Simon Glass <sjg@chromium.org>
Signed-off-by: Tom Rini <trini@ti.com>