power: regulator: gpio-regulator: protect count value

Update the size of states_array to avoid overflow for
dev_pdata->voltages[j] and dev_pdata->states[j].

As the size of array is GPIO_REGULATOR_MAX_STATES, the size of
states_array is limited by GPIO_REGULATOR_MAX_STATES * 2 = 4
instead of 8 previously.

The value of the "count" variable is limited by the third parameter of
fdtdec_get_int_array_count.

Signed-off-by: Patrick Delaunay <patrick.delaunay@st.com>
Reviewed-by: Simon Glass <sjg@chromium.org>