bus: moxtet: Mark the irq as shared

The Turris Mox shares the moxtet IRQ with various devices on the board,
so mark the IRQ as shared in the driver as well.

Without this loading the module will fail with:
  genirq: Flags mismatch irq 40. 00002002 (moxtet) vs. 00002080 (mcp7940x)

Signed-off-by: Sjoerd Simons <sjoerd@collabora.com>
Cc:  <stable@vger.kernel.org> # v6.2+
Reviewed-by: Marek Behún <kabel@kernel.org>
Signed-off-by: Gregory CLEMENT <gregory.clement@bootlin.com>