MIPS: lantiq: xway: add prototype for ltq_get_cp1_base()

ltq_get_cp1_base() is an exported function, we must define
its prototype on header file. Fix warning:

arch/mips/lantiq/xway/vmmc.c:22:15: error: no previous prototype for 'ltq_get_cp1_base' [-Werror=missing-prototypes]
   22 | unsigned int *ltq_get_cp1_base(void)
      |               ^~~~~~~~~~~~~~~~

Signed-off-by: Shiji Yang <yangshiji66@outlook.com>
Signed-off-by: Thomas Bogendoerfer <tsbogend@alpha.franken.de>