gpio: xtensa: fix build when XCHAL_HAVE_CP is 0

In xtensa coprocessors may exist without coprocessor context, i.e. they cannot
be disabled/enabled. In this case the RSR_CPENABLE/WSR_CPENABLE are undefined,
thus breaking the build. Fix the build by adding dummy versions of
enable_cp/disable_cp in this case.

Reported-by: Fengguang Wu <fengguang.wu@intel.com>
Signed-off-by: Baruch Siach <baruch@tkos.co.il>
Signed-off-by: Linus Walleij <linus.walleij@linaro.org>