Definitions for GPIO registers 8, 9, 10, 11, 12 and 13 are added into
the register header file.

- DA9052_GPIO_8_9_REG    25
- DA9052_GPIO_10_11_REG  26
- DA9052_GPIO_12_13_REG  27

A modification is also made to the MFD core code to define these registers
as readable and writable. The functions for da9052_reg_readable() and
da9052_reg_writeable() have had their case statements altered to include
these new registers.
Signed-off-by: Steve Twiss <stwiss.opensource@diasemi.com>
Signed-off-by: Lee Jones <lee.jones@linaro.org>