arm: prima2: use of_platform_populate instead of of_platform_bus_probe

use of_platform_populate to populate platform devices, it is suitable
for creating devices which are children of the root node, then makes
things like pcm work.

this patch removes sirfsoc_mach_init() function and lets arm core do
of_platform_populate:

    static int __init customize_machine(void)
    {
            /*
             * customizes platform devices, or adds new ones
             * On DT based machines, we fall back to populating the
             * machine from the device tree, if no callback is provided,
             * otherwise we would always need an init_machine callback.
             */
            if (machine_desc->init_machine)
                    machine_desc->init_machine();
            else
                    of_platform_populate(NULL, of_default_bus_match_table,
                                            NULL, NULL);
            return 0;
    }

Signed-off-by: Barry Song <Baohua.Song@csr.com>
Signed-off-by: Rongjun Ying <Rongjun.Ying@csr.com>