macb: Add support for SiFive FU540-C000

The management IP block is tightly coupled with the Cadence MACB IP
block on the FU540, and manages many of the boundary signals from the
MACB IP. This patch only controls the tx_clk input signal to the MACB
IP. Future patches may add support for monitoring or controlling other
IP boundary signals.

Signed-off-by: Yash Shah <yash.shah@sifive.com>
Signed-off-by: David S. Miller <davem@davemloft.net>