mfd: da9052: Constify spi_device_id

spi_device_id are not supposed to change at runtime. All functions
working with spi_device_id provided by <linux/i2c.h> work with
const spi_device_id. So mark the non-const structs as const.

Signed-off-by: Arvind Yadav <arvind.yadav.cs@gmail.com>
Signed-off-by: Lee Jones <lee.jones@linaro.org>