mailbox: Switch back to struct platform_driver::remove()

After commit 0edb555a ("platform: Make platform_driver::remove()
return void") .remove() is (again) the right callback to implement for
platform drivers.

Convert all platform drivers below drivers/mailbox to use .remove(),
with the eventual goal to drop struct platform_driver::remove_new(). As
.remove() and .remove_new() have the same prototypes, conversion is done
by just changing the structure member name in the driver initializer.

Make a few indentions consistent while touching these struct
initializers.

Signed-off-by: Uwe Kleine-König <u.kleine-koenig@baylibre.com>
Reviewed-by: AngeloGioacchino Del Regno <angelogioacchino.delregno@collabora.com>
Acked-by: Chen-Yu Tsai <wens@csie.org>
Signed-off-by: Jassi Brar <jassisinghbrar@gmail.com>