docs: qemu/sifive_u: miscellaneous documentation fixes

- OpenSBI build target should be: qemu/sifive_u
- U-Boot config name should be: sifive_fu540_defconfig

Signed-off-by: Bin Meng <bmeng.cn@gmail.com>
Reviewed-by: Atish Patra <atish.patra@wdc.com>