If we build a kernel with PM_SUSPEND set and no PM_SLEEP,
we get a build warning in the xhci-plat driver about unused
functions.

To fix this, use "#ifdef CONFIG_PM_SLEEP", like we do in most
other drivers nowadays.
Signed-off-by: Arnd Bergmann <arnd@arndb.de>
Cc: Mathias Nyman <mathias.nyman@intel.com>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>