x86: apl: Use cpu_x86_get_count() for cpu_ops.get_count

Use cpu_x86_get_count() to read the number of cores.

cpu_x86_get_count() reads the number of CPUs from the device tree.
Using this function we can support multiple Apollo Lake
variants, e.g.: E3940 (4 cores) and E3930 (2 cores).

This was tested on the E3940 and E3930 Apollo Lake variants.

Signed-off-by: Wolfgang Wallner <wolfgang.wallner@br-automation.com>
Reviewed-by: Bin Meng <bmeng.cn@gmail.com>