docs/system: riscv: Add documentation for sifive_u machine

This adds detailed documentation for RISC-V `sifive_u` machine,
including the following information:

- Supported devices
- Hardware configuration information
- Boot options
- Machine-specific options
- Running Linux kernel
- Running VxWorks kernel
- Running U-Boot, and with an alternate configuration

Signed-off-by: Bin Meng <bin.meng@windriver.com>
Reviewed-by: Alistair Francis <alistair.francis@wdc.com>
Reviewed-by: Palmer Dabbelt <palmerdabbelt@google.com>
Message-id: 20210126060007.12904-10-bmeng.cn@gmail.com
Signed-off-by: Alistair Francis <alistair.francis@wdc.com>

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