doc: Add vendor filter for hw feature generation

The hw feature generation takes a long time. The HW_FEATURES_TURBO_MODE
option completely disables hw feature generation. Add a new option
HW_FEATURES_VENDOR_FILTER to be able to selectively enable hw feature
generation only for a given list of vendors. This option is useful
when working on board documentation pages.

Signed-off-by: Aksel Skauge Mellbye <aksel.mellbye@silabs.com>