mfd: wm8998: Fixup register defaults/readables

Remove defaults for a bunch of volatile registers and remove
ARIZONA_CTRL_IF_SPI_CFG_1 from the readable list since it doesn't exist
on wm8998 which is I2C only.

Signed-off-by: Charles Keepax <ckeepax@opensource.wolfsonmicro.com>
Signed-off-by: Lee Jones <lee.jones@linaro.org>