spi: amlogic-spisg: Make sure clk_init_data is fully initialized

In the Linux kernel, the following vulnerability has been resolved:

spi: amlogic-spisg: Make sure clk_init_data is fully initialized

The clk_init_data structure contains several mutually-exclusive members
for different methods to specify the possible parents of a clock,
prompting drivers to initialize only the members they need. However,
not initializing all members may cause subtle issues, which are only
exposed when CONFIG_INIT_STACK_ALL_PATTERN or CONFIG_INIT_STACK_NONE is
enabled.

aml_spisg_clk_init() fills in init.parent_data, and assumes that
init.parent_names is NULL. However, the latter in uninitialized, and
thus may cause a crash.

Make sure all members are fully initialized, to fix such bugs, and to
avoid future breakage when converting drivers to a different method for
specifying the parents.