mfd: Add TPS659038 documentation under Palmas

Add TPS659038 Device Tree documentation.

Acked-by: Samuel Ortiz <sameo@linux.intel.com>
Signed-off-by: J Keerthy <j-keerthy@ti.com>
Signed-off-by: Lee Jones <lee.jones@linaro.org>
Signed-off-by: Samuel Ortiz <sameo@linux.intel.com>