riscv: sifive_u: Fix broken GEM support

riscv: sifive_u: Fix broken GEM support

At present the GEM support in sifive_u machine is seriously broken.
The GEM block register base was set to a weird number (0x100900FC),
which for no way could work with the cadence_gem model in QEMU.

Not like other GEM variants, the FU540-specific GEM has a management
block to control 10/100/1000Mbps link speed changes, that is mapped
to 0x100a0000. We can simply map it into MMIO space without special
handling using create_unimplemented_device().

Update the GEM node compatible string to use the official name used
by the upstream Linux kernel, and add the management block reg base
& size to the <reg> property encoding.

Tested with upstream U-Boot and Linux kernel MACB drivers.

Signed-off-by: Bin Meng <bmeng.cn@gmail.com>
Reviewed-by: Alistair Francis <alistair.francis@wdc.com>
Signed-off-by: Palmer Dabbelt <palmer@sifive.com>