spi: McSPI off-mode support

Add context save/restore feature to McSPI driver.

Signed-off-by: Hemanth V <hemanthv@ti.com>
Reviewed-by: Aaro Koskinen <Aaro.Koskinen@nokia.com>
Signed-off-by: Kevin Hilman <khilman@deeprootsystems.com>
Signed-off-by: David Brownell <dbrownell@users.sourceforge.net>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>