clk: fix clk-gpio.c with optional clock= DT property

When the clock DT property is not given, of_clk_get_parent_count()
returns -ENOENT, which then tries to allocate -2 x 4 bytes of memory,
which of course fails, causing the whole driver to fail to create
the clock.

This causes the SolidRun platforms to fail probing the SDHCI1 interface
which is connected to the WiFi.

Fix this by detecting errno codes, skipping the allocation, and fixing
of_clk_gpio_gate_delayed_register_get() to handle a NULL parent_names
array.

Fixes: 80eeb1f0 ("clk: add gpio controlled clock multiplexer")
Signed-off-by: Russell King <rmk+kernel@arm.linux.org.uk>
Signed-off-by: Michael Turquette <mturquette@baylibre.com>