mfd: arizona: Lower ARIZONA_MAX_CORE_SUPPLIES to 2

There are no Arizona devices with 3 core supplies but we define a fix
array with space for 3 core supplies. Lower the ARIZONA_MAX_CORE_SUPPLIES
define to 2, to save a few bytes.

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