dpll: add dpll_device op to get supported modes

dpll: add dpll_device op to get supported modes

Currently, the DPLL subsystem assumes that the only supported mode is
the one currently active on the device. When dpll_msg_add_mode_supported()
is called, it relies on ops->mode_get() and reports that single mode
to userspace. This prevents users from discovering other modes the device
might be capable of.

Add a new callback .supported_modes_get() to struct dpll_device_ops. This
allows drivers to populate a bitmap indicating all modes supported by
the hardware.

Update dpll_msg_add_mode_supported() to utilize this new callback:

* if ops->supported_modes_get is defined, use it to retrieve the full
  bitmap of supported modes.
* if not defined, fall back to the existing behavior: retrieve
  the current mode via ops->mode_get and set the corresponding bit
  in the bitmap.

Finally, iterate over the bitmap and add a DPLL_A_MODE_SUPPORTED netlink
attribute for every set bit, accurately reporting the device's capabilities
to userspace.

Reviewed-by: Vadim Fedorenko <vadim.fedorenko@linux.dev>
Signed-off-by: Ivan Vecera <ivecera@redhat.com>
Link: https://patch.msgid.link/20260114122726.120303-2-ivecera@redhat.com
Signed-off-by: Jakub Kicinski <kuba@kernel.org>