dpll: zl3073x: add reference priority to zl3073x_chan

dpll: zl3073x: add reference priority to zl3073x_chan

Cache the ZL_REG_DPLL_REF_PRIO registers in the zl3073x_chan cfg group.
These mailbox-based registers store per-reference priority values
(4 bits each, P/N packed) used for automatic reference selection.

Add ref_prio[] array to struct zl3073x_chan and provide inline helpers
zl3073x_chan_ref_prio_get(), zl3073x_chan_ref_prio_set(), and
zl3073x_chan_ref_is_selectable() for nibble-level access and priority
queries. Extend state_fetch and state_set with DPLL mailbox operations
to read and write the priority registers.

Replace the ad-hoc zl3073x_dpll_ref_prio_get/set functions in dpll.c
with the cached state pattern, removing direct mailbox access from
the DPLL layer. This also simplifies pin registration since reading
priority from cached state cannot fail.

Remove the pin->selectable flag from struct zl3073x_dpll_pin and
derive the selectable state from the cached ref priority via
zl3073x_chan_ref_is_selectable(), eliminating a redundant cache.

Inline zl3073x_dpll_selected_ref_set() into
zl3073x_dpll_input_pin_state_on_dpll_set(), unifying all manual and
automatic mode paths to commit changes through a single
zl3073x_chan_state_set() call at the end of the function.

Move hardware limit constants from core.h to regs.h so that chan.h
can reference ZL3073X_NUM_REFS for the ref_prio array size.

Signed-off-by: Ivan Vecera <ivecera@redhat.com>
Link: https://patch.msgid.link/20260315174224.399074-6-ivecera@redhat.com
Signed-off-by: Jakub Kicinski <kuba@kernel.org>