phy: miphy28lp: Pass sysconfig register offsets via syscfg dt property.

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

,
update the miphy28lp phy driver to access sysconfig register offsets via
syscfg dt property.

This is because the reg property should not be mixing address spaces like
it does currently for miphy28lp. This change then also aligns us to how other
platforms such as keystone and bcm7445 pass there syscon offsets via DT.

I have updated the miphy28lp phy driver same way as Peter's implementation.

Signed-off-by: Gabriel Fernandez <gabriel.fernandez@linaro.org>
Signed-off-by: Kishon Vijay Abraham I <kishon@ti.com>