net: mscc: ocelot: simplify register access for PLL5 configuration

Since HSIO address space can be accessed by different drivers, let's
simplify the register address definitions so that it can be easily used
by all drivers and put the register address definition in the
include/soc/mscc/ocelot_hsio.h header file.

Reviewed-by: Florian Fainelli <f.fainelli@gmail.com>
Acked-by: Alexandre Belloni <alexandre.belloni@bootlin.com>
Signed-off-by: Quentin Schulz <quentin.schulz@bootlin.com>
Signed-off-by: David S. Miller <davem@davemloft.net>