broadcom: Isolate phy dsp accesses

This patch consolidates the code that requires the SMDSP clock to be
enabled into a single function that (hopefully) makes the dependency
obvious.

Signed-off-by: Matt Carlson <mcarlson@broadcom.com>
Reviewed-by: Michael Chan <mchan@broadcom.com>
Signed-off-by: David S. Miller <davem@davemloft.net>