phy: phy-stih407-usb: Pass sysconfig register offsets via syscfg property.

Based on Arnds review comments here https://lkml.org/lkml/2014/11/13/161

,
update the phy driver to not use the reg property to access the sysconfig
register offsets.

This is because other phy's (miphy28, miphy365) have a combination of
memory mapped registers and sysconfig control regs, and we shouldn't
be mixing address spaces in the reg property. In addition we would
ideally like the sysconfig offsets to be passed via DT in a uniform way.

This new method will also allow us to support devices which have sysconfig
registers in different banks more easily and it is also analagous to how
keystone and bcm7745 platforms pass there syscon offsets in DT.

This breaks DT compatibility, but this platform is considered WIP, and
is only used by a few developers who are upstreaming support for it.

Signed-off-by: Peter Griffin <peter.griffin@linaro.org>
Reviewed-by: Arnd Bergmann <arnd@arndb.de>
Signed-off-by: Maxime Coquelin <maxime.coquelin@st.com>
Signed-off-by: Kishon Vijay Abraham I <kishon@ti.com>