Not all architectures(i.e. i.MXRT) support mxc_get_clock() and use DM_CLK
instead. So building could result in failure due to missing
mxc_get_clock().

Make if(CONFIG_IS_ENABLED(CLK)) an #if statement.
Signed-off-by: NGiulio Benetti <giulio.benetti@benettiengineering.com>