platform: Rename sifive/hifive_u540 to sifive/fu540

We rename sifive/hifive_u540 platform support to sifive/fu540
to match the SoC name which is SiFive FU540 (as-per user manual).

In fact, we can expect no (or very minimal) board specific
quirks in sifive/fu540 platform support. These board specific
quirks can be turned on/off in-future using PLATFORM_<xyz>
make command-line option.

Signed-off-by: Anup Patel <anup.patel@wdc.com>
Signed-off-by: Atish Patra <atish.patra@wdc.com>