ARM: rockchip: rock: Correct test to use CONFIG_IS_ENABLED not defined

While it is likely that this entire case is superfluous and can be
removed, correct the test now to match what is in rockchip-common.h and
makes sense based on context of the code.  Otherwise we get a large
number of warnings.

Reviewed-by: Philipp Tomsich <philipp.tomsich@theobroma-systems.com>
Signed-off-by: Tom Rini <trini@konsulko.com>