Rename sifive_fu740 -> sifive_unmatched, sifive_fu540 -> sifive_unleashed

[davidlt]

This cleanup already happened on U-Boot side. FU540 and FU740 are SoC names, not the actual boards/platforms. For example, the reset implemented in sifive_fu740.c depends on DA9063 PMIC (that's board specific details).

It's a not a big issue considering that for each SoC there is a single board, but it's confusing.