Blackfin: twi: Add missing __iomem annotation

Add a missing __iomem to the bfin_twi_iface struct's regs_base field. This fixes
the following sparse warnings:

Fixes the following sparse warnings:
	drivers/i2c/busses/i2c-bfin-twi.c:641:26: warning: incorrect type in assignment (different address spaces)
	drivers/i2c/busses/i2c-bfin-twi.c:641:26:    expected struct bfin_twi_regs 	*regs_base
	drivers/i2c/busses/i2c-bfin-twi.c:641:26:    got void [noderef] <asn:2>*
	drivers/i2c/busses/i2c-bfin-twi.c:715:22: warning: incorrect type in argument 1 (different address spaces)
	drivers/i2c/busses/i2c-bfin-twi.c:715:22:    expected void [noderef] <asn:2>*addr
	drivers/i2c/busses/i2c-bfin-twi.c:715:22:    got struct bfin_twi_regs *regs_base
	drivers/i2c/busses/i2c-bfin-twi.c:732:22: warning: incorrect type in argument 1 (different address spaces)
	drivers/i2c/busses/i2c-bfin-twi.c:732:22:    expected void [noderef] <asn:2>*addr
	drivers/i2c/busses/i2c-bfin-twi.c:732:22:    got struct bfin_twi_regs *regs_base

Signed-off-by: Lars-Peter Clausen <lars@metafoo.de>
Signed-off-by: Bob Liu <lliubbo@gmail.com>