powerpc/pseries: Move hvc_vio_init_early() prototype to shared location

Fixes the following W=1 kernel build warning(s):

 drivers/tty/hvc/hvc_vio.c:385:13: warning: no previous prototype for ‘hvc_vio_init_early’
 385 | void __init hvc_vio_init_early(void)
 | ^~~~~~~~~~~~~~~~~~

Signed-off-by: Lee Jones <lee.jones@linaro.org>
Signed-off-by: Michael Ellerman <mpe@ellerman.id.au>
Link: https://lore.kernel.org/r/20210303124603.3150175-1-lee.jones@linaro.org