net: phy: realtek: add utility functions to read/write page addresses

Realtek PHYs implement the concept of so-called "extension pages". The
reason for this is probably because these PHYs expose more registers
than available in the standard address range.
After all read/write operations on such a page are done the driver
should switch back to page 0 where the standard MII registers (such as
MII_BMCR) are available.

When referring to such a register the datasheets of RTL8211E and
RTL8211F always specify:
- the page / "ext. page" which has to be written to RTL821x_PAGE_SELECT
- an address (sometimes also called reg)

These new utility functions make the existing code easier to read since
it removes some duplication (switching back to page 0 is done within the
new helpers for example).

No functional changes are intended.

Signed-off-by: Martin Blumenstingl <martin.blumenstingl@googlemail.com>
Reviewed-by: Andrew Lunn <andrew@lunn.ch>
Signed-off-by: David S. Miller <davem@davemloft.net>