pinctrl: generic: scan for "pins" and "groups" properties in sub-nodes

pinctrl: generic: scan for "pins" and "groups" properties in sub-nodes

In cases where the pins and groups definitions are in a sub-node, as:

	uart_a {
		mux {
			groups = "uart_tx_a", "uart_rx_a";
			function = "uart_a";
		};
	};

pinctrl_generic_set_state_subnode() returns an error for the top-level
node and pinctrl_generic_set_state() fails. Instead, return success so
that the child nodes are tried.

Signed-off-by: Beniamino Galvani <b.galvani@gmail.com>
Reviewed-by: Simon Glass <sjg@chromium.org>
Reviewed-by: Masahiro Yamada <yamada.masahiro@socionext.com>