phy: cadence: Sierra: Add support for skipping configuration

In some cases, a single SerDes instance can be shared between two different
processors, each using a separate link. In these cases, the SerDes
configuration is done in an earlier boot stage. Therefore, add support to
skip reconfiguring, if it is was already configured beforehand.

Signed-off-by: Aswath Govindraju <a-govindraju@ti.com>