gpio: aggregator: Wrap access to gpiochip_fwd.tmp[]

The tmp[] member of the gpiochip_fwd structure is used to store both the
temporary values bitmap and the desc pointers for operations on multiple
GPIOs.  As both are arrays with sizes unknown at compile-time, accessing
them requires offset calculations, which are currently duplicated in
gpio_fwd_get_multiple() and gpio_fwd_set_multiple().

Introduce (a) accessors for both arrays and (b) a macro to calculate the
needed storage size.  This confines the layout of the tmp[] member into
a single spot, to ease maintenance.

Signed-off-by: Geert Uytterhoeven <geert+renesas@glider.be>
Signed-off-by: Bartosz Golaszewski <brgl@bgdev.pl>