i2c: add debug message for detected HostNotify alerts

Setting up HostNotify can be tricky. Support debugging by stating
when a HostNotify alert was received independent of the irq being
mapped. Especially useful with the in-kernel i2c testunit. Update
documentation as well.

Signed-off-by: Wolfram Sang <wsa+renesas@sang-engineering.com>
Reviewed-by: Andi Shyti <andi.shyti@kernel.org>