clk: renesas: rzv2h: Skip monitor checks for external clocks

For module clocks whose parent mux may select an external source, bypass
the normal monitor (CLK_MON) register check when the external clock is
active. Introduce a new `ext_clk_mux_index` in `struct rzv2h_mod_clk` and
`struct mod_clock`, and detect the current mux index in
`rzv2h_mod_clock_is_enabled()` to disable monitoring if it matches the
external source index.

Provide the `DEF_MOD_MUX_EXTERNAL()` macro for declaring external-source
module clocks, and populate the `ext_clk_mux_index` field in
`rzv2h_cpg_register_mod_clk()`.

Signed-off-by: Lad Prabhakar <prabhakar.mahadev-lad.rj@bp.renesas.com>
Reviewed-by: Geert Uytterhoeven <geert+renesas@glider.be>
Link: https://lore.kernel.org/20250509160121.331073-2-prabhakar.mahadev-lad.rj@bp.renesas.com


Signed-off-by: Geert Uytterhoeven <geert+renesas@glider.be>