ahci: st: Update the ahci_st DT documentation

As part of testing ahci_st driver working on stih407 I noticed
several things wrong in the DT documentation: -

1) Compatible string doesn't match the driver code
2) pwr-rst reset isn't documented (but exists in the driver)
3) some whitespace issues (spaces not tabs)

Also add in a stih407 family example into the doc.

Signed-off-by: Peter Griffin <peter.griffin@linaro.org>
Acked-by: Lee Jones <lee.jones@linaro.org>
Acked-by: Maxime Coquelin <maxime.coquelin@st.com>
Signed-off-by: Tejun Heo <tj@kernel.org>