phy: miphy28lp: fix sparse warnings

Add missing __iomem annotation to the base address
so Sparse doesn't complain.

Signed-off-by: Felipe Balbi <balbi@ti.com>
Acked-by: Maxime Coquelin <maxime.coquelin@st.com>
Signed-off-by: Kishon Vijay Abraham I <kishon@ti.com>