Xilinx: ARM: cpufreq: Fix potential NULL ptr dereference

If no OPPs exist, creating the freq_table fails. Error
handling in this case worked for the first core to
execute the cpufreq init routine. But a following core
would have unconditionally called
cpufreq_frequency_table_cpuinfo() causing a NULL pointer
dereference.

A corresponding fix for OMAP had been applied to mainline:
1b865214

Signed-off-by: Soren Brinkmann <soren.brinkmann@xilinx.com>