85xx/fsl-sata: Use is_serdes_configured() to determine if SATA is enabled

On the MPC85xx platform if we have SATA its connected on SERDES.
Determing if SATA is enabled via sata_initialize should not be board
specific and thus we move it out of the MPC8536DS board code.

Additionally, now that we have is_serdes_configured() we can determine
if the given SATA port is enabled and error out if its not in the
driver.

Signed-off-by: Kumar Gala <galak@kernel.crashing.org>