c6x: fix platforms/plldata.c get_coreid build error

Fix build error reported by the 0day bot by including the header
file for that macro.

Fixes this build error: (should fix; not tested)
arch/c6x/platforms/plldata.c: In function 'c6472_setup_clocks':
arch/c6x/platforms/plldata.c:279:33: error: implicit declaration of function 'get_coreid'; did you mean 'get_order'? [-Werror=implicit-function-declaration]
      c6x_core_clk.parent = &sysclks[get_coreid() + 1];

Reported-by: Fengguang Wu <fengguang.wu@intel.com>
Cc: Mark Salter <msalter@redhat.com>
Cc: Aurelien Jacquiot <jacquiot.aurelien@gmail.com>
Cc: linux-c6x-dev@linux-c6x.org
Cc: Ingo Molnar <mingo@kernel.org>

Signed-off-by: Randy Dunlap <rdunlap@infradead.org>
Signed-off-by: Mark Salter <msalter@redhat.com>