mfd : Fix dbx500 compilation error

The ux500 default config enables the db5500 and the db8500.
The incoming cpuidle driver uses the 'prcmu_enable_wakeups'
and the 'prcmu_set_power_state' functions but these ones
are defined but not implemented for the db5500, leading to
an unresolved symbol error at link time. In order to compile,
we have to disable the db5500 support which is not acceptable
for the default config.

I noticed there are also some other functions which are
defined but not implemented.

This patch fix this by removing the functions definitions
and move out of the config section the empty functions which
are normally used when the DB550 config is disabled.
Only the functions which are not implemented are concerned
by this modification.

Signed-off-by: Daniel Lezcano <daniel.lezcano@linaro.org>
Acked-by: Linus Walleij <linus.walleij@linaro.org>
Signed-off-by: Samuel Ortiz <sameo@linux.intel.com>