serial: stm32: Fix AARCH64 compilation warnings

When building with AARCH64 defconfig, we got warnings, fix them
by using registers base address defined as void __iomem * instead of
fdt_addr_t.

Signed-off-by: Patrice Chotard <patrice.chotard@st.com>
Signed-off-by: Patrice Chotard <patrice.chotard@foss.st.com>
Reviewed-by: Patrick Delaunay <patrick.delaunay@foss.st.com>