aspeed: Add support for Clocks needed by MACs

aspeed: Add support for Clocks needed by MACs

Add support for clocks needed by MACs to ast2500 clock driver.
The clocks are D2-PLL, which is used by both MACs and PCLK_MAC1 and
PCLK_MAC2 for MAC1 and MAC2 respectively.

The rate of D2-PLL is hardcoded to 250MHz -- the value used in Aspeed
SDK. It is not entirely clear from the datasheet how this clock is used
by MACs, so not clear if the rate would ever need to be different. So,
for now, hardcoding it is probably safer.

The rate of PCLK_MAC{1,2} is chosen based on MAC speed selected through
hardware strapping.

So, the network driver would only need to enable these clocks, no need
to configure the rate.

Signed-off-by: Maxim Sloyko <maxims@google.com>
Reviewed-by: Simon Glass <sjg@chromium.org>