blackfin: Fix build error

Fix

include/asm-generic/io.h: In function 'readb':
include/asm-generic/io.h:113:2: error:
	implicit declaration of function 'bfin_read8'
include/asm-generic/io.h: In function 'readw':
include/asm-generic/io.h:121:2: error:
	implicit declaration of function 'bfin_read16'
include/asm-generic/io.h: In function 'readl':
include/asm-generic/io.h:129:2: error:
	implicit declaration of function 'bfin_read32'
include/asm-generic/io.h: In function 'writeb':
include/asm-generic/io.h:147:2: error:
	implicit declaration of function 'bfin_write8'
include/asm-generic/io.h: In function 'writew':
include/asm-generic/io.h:155:2: error:
	implicit declaration of function 'bfin_write16'
include/asm-generic/io.h: In function 'writel':
include/asm-generic/io.h:163:2: error:
	implicit declaration of function 'bfin_write32'

Reported-by: Geert Uytterhoeven <geert@linux-m68k.org>
Fixes: 1a3372bc

 ("blackfin: io: define __raw_readx/writex with
	bfin_readx/writex")
Cc: Steven Miao <realmz6@gmail.com>
Signed-off-by: Guenter Roeck <linux@roeck-us.net>