Group both the HSW/LPT SBI interface and VLV IOSF sideband register
accessor functions into a new file. No functional changes.

v2: also move intel_sbi_{read,write} (Daniel)
Signed-off-by: NJani Nikula <jani.nikula@intel.com>
Reviewed-by: NJesse Barnes <jbarnes@virtuousgeek.org>
Signed-off-by: NDaniel Vetter <daniel.vetter@ffwll.ch>