xtensa: fix warning '"CONFIG_OF" is not defined'

The warning only shows up when building MMUv3 configuration with OF
support disabled.

Signed-off-by: Max Filippov <jcmvbkbc@gmail.com>
xtensa: export ccount_freq

Now that ccount_freq is used in udelay and ndelay it needs to be
exported in order to be available to modules.

Signed-off-by: Max Filippov <jcmvbkbc@gmail.com>