headers_check fix: powerpc, bootx.h

fix the following 'make headers_check' warnings:

  usr/include/asm-powerpc/bootx.h:12: include of <linux/types.h> is preferred over <asm/types.h>
  usr/include/asm-powerpc/bootx.h:57: found __[us]{8,16,32,64} type without #include <linux/types.h>

Signed-off-by: Jaswinder Singh Rajput <jaswinderrajput@gmail.com>