Merge https://gitlab.denx.de/u-boot/custodians/u-boot-riscv

- Sipeed Maix support S-mode.
- Provide command sbi.
- Use fdtdec_get_addr_size_auto_parent to get fu540 cache base address.
- Fix a compiler error with CONFIG_SPL_SMP=n.
- Fix sifive ram driver 32 compiler warnings.
- Fix kendryte/pll.h redefine nop() warning.