pwm: lpss: Clarify the bypass member semantics in struct pwm_lpss_boardinfo

Instead of an odd comment, cite the documentation, which says more clearly
what's going on with the programming flow on some of the Intel SoCs.

Signed-off-by: Andy Shevchenko <andriy.shevchenko@linux.intel.com>
Reviewed-by: Mika Westerberg <mika.westerberg@linux.intel.com>