clk: sifive: Add clock driver for GEMGXL MGMT

This adds a clock driver to support the GEMGXL management IP block
found in FU540 SoCs to control GEM TX clock operation mode for
10/100/1000 Mbps.

Signed-off-by: Bin Meng <bmeng.cn@gmail.com>
Reviewed-by: Lukas Auer <lukas.auer@aisec.fraunhofer.de>
Tested-by: Lukas Auer <lukas.auer@aisec.fraunhofer.de>