ARM: ux500: tidy up pin sleep modes

This named the sleep mode pin configurations as *slpm* rather
than *sleep* to correspond better with the settings from the
datasheet. It also defines an optional sleep mode for the SPI
controller SPI2.

Acked-by: Arnd Bergmann <arnd@arndb.de>
Signed-off-by: Linus Walleij <linus.walleij@linaro.org>