reset: berlin: support module build

Make reset-berlin driver to be tristate module, support to build as
a module, this is useful for GKI.

Partially reverts commit ed4dba99

 ("reset: berlin: make it
explicitly non-modular")

Signed-off-by: Jisheng Zhang <Jisheng.Zhang@synaptics.com>
Link: https://lore.kernel.org/r/20210607181015.5b8d3711@xhacker.debian


Signed-off-by: Philipp Zabel <p.zabel@pengutronix.de>