Add a hook to allow board-specific PIO mode setting.

This patch adds a hook whereby a board-specific routine can be called to
configure hardware for a PIO mode.  The prototype for the board-specific
routine is:

	int inline ide_set_piomode(int pio_mode)

ide_set_piomode should be prepared to configure hardware for a pio_mode
between 0 and 6, inclusive.  It should return 0 on success or 1 on failure.

Signed-off-by: Steven A. Falco <sfalco@harris.com>