spi/pl022: mark driver non-experimental

This driver has no TODO, and is now used on several platforms:
ARM, U300, Ux500, SPEAr and more. So drop the EXPERIMENTAL
requirement.

Signed-off-by: Linus Walleij <linus.walleij@stericsson.com>
Signed-off-by: Grant Likely <grant.likely@secretlab.ca>