Add bcm_clk_stub, a minimal fixed-frequency clknode used as a
placeholder for clock sources that are not yet fully modelled (e.g.
gnd, testdebug0/1, pllh_aux on BCM2835).

The stub node reports a fixed frequency supplied at registration time
and rejects any set_freq request that would change it. This allows
the peripheral clock mux to resolve all parent names at clkdom_finit()
time without requiring every source to have a real implementation.

bcm_cprman_clk_data gains a stub_clks array; bcm_cprman_attach
registers stub clocks first so they are available as parents when PLLs
and peripheral clocks are registered.

Signed-off-by: perdixky <3293789706@qq.com>