reset: meson: make it possible to build as a module

In order to reduce the kernel Image size on multi-platform distributions,
make it possible to build the reset controller driver as a module.

This partially reverts commit 8290924e ("reset: meson: make it
explicitly non-modular").

Signed-off-by: Neil Armstrong <narmstrong@baylibre.com>
Reviewed-by: Kevin Hilman <khilman@baylibre.com>
Tested-by: Kevin Hilman <khilman@baylibre.com>
Reviewed-by: Martin Blumenstingl <martin.blumenstingl@googlemail.com>
Signed-off-by: Philipp Zabel <p.zabel@pengutronix.de>