stmmac: Add an optional register interface clock

The DWMAC block on certain SoCs (such as IMG Pistachio) have a second
clock which must be enabled in order to access the peripheral's
register interface, so add support for requesting and enabling an
optional "pclk".

Signed-off-by: Andrew Bresticker <abrestic@chromium.org>
Cc: James Hartley <james.hartley@imgtec.com>
Cc: Arnd Bergmann <arnd@arndb.de>
Signed-off-by: David S. Miller <davem@davemloft.net>