mfd: Update abstract dbx500 interface

This prefixes a number of accessor functions with db8500_* since
they are DB8500-specific and we need to move to this naming
scheme.

We also replace numerous instances of machine_is() with cpu_is()
which covers the right type of ASICs rather than entire machines
i.e. boards.

Signed-off-by: Mattias Nilsson <mattias.i.nilsson@stericsson.com>
Signed-off-by: Linus Walleij <linus.walleij@linaro.org>
Signed-off-by: Samuel Ortiz <sameo@linux.intel.com>