sh: boot word / mode pin support V2

Add mode pin support for the SuperH architecture V2.

With this patch applied the board code can add their
own function to export the cpu mode pin configuration.
In most cases this will be a constant bitmap, but
boards that allow reading this from a register can
instead read out the pin state from hardware.

The code warns if a pin is tested but no board specific
mode pin function has been provided.

Signed-off-by: Magnus Damm <damm@igel.co.jp>
Signed-off-by: Paul Mundt <lethal@linux-sh.org>