iio: adc: ade9000: fix NULL pointer dereference in clkout registration

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

iio: adc: ade9000: fix NULL pointer dereference in clkout registration

ade9000_setup_clkout() passes NULL as the register address when
registering a divider clock. During clock registration, the common
clock framework calls clk_divider_recalc_rate(), which dereferences
the address through readl(). As a result, probing an ADE9000 configured
as a clock provider with an external input clock crashes.

CLKOUT passes CLKIN through without changing its rate. Register it as
a 1:1 fixed-factor clock, which does not require register access.

This change does not affect the configuration using the internal clock,
for which the driver does not register a clock provider.