errno: Allow errno_str() to be used without CONFIG_ERRNO_STR

errno: Allow errno_str() to be used without CONFIG_ERRNO_STR

The pmic framework uses errno_str() and this requires board that use it to
enable CONFIG_ERRNO_STR to avoid a build error. Update the header to provide
a NULL error message when CONFIG_ERRNO_STR is not defined, and fix the build
error.

This will show as "(null)" when U-Boot prints it.

Signed-off-by: Simon Glass <sjg@chromium.org>
Signed-off-by: Tom Warren <twarren@nvidia.com>