clk: clk-gpio: Allow GPIO to sleep in set/get_parent

When changing or retrieving clock parents, the caller is in a sleepable
state (like prepare) so the GPIO operation need not be atomic. Replace
gpiod_{g|s}et_value with gpiod_{g|s}et_value_cansleep in the {g|s}et_parent
calls for the GPIO based clock mux.

This fixes a "slowpath" warning when the GPIO controller is an I2C expander
or something similar.

Signed-off-by: Mike Looijmans <mike.looijmans@topic.nl>
Signed-off-by: Stephen Boyd <sboyd@kernel.org>