Backport subject : 'RISC-V SBI debug console extension support'

[yechao-w]

反合上游补丁，支持SBI DBCN extension。包括KVM虚拟机支持部分。

补丁集：

f503b16 RISC-V: Add stubs for sbi_console_putchar/getchar()
f43fabf RISC-V: Add SBI debug console helper routines
c77bf36 tty/serial: Add RISC-V SBI debug console based earlycon
88ead68 tty: Add SBI debug console support to HVC SBI driver
50942ad RISC-V: Enable SBI based earlycon support

dadf788 RISC-V: Add defines for SBI debug console extension
b88e87a RISC-V: KVM: Change the SBI specification version to v2.0
56d8a38 RISC-V: KVM: Allow some SBI extensions to be disabled by default
c667ad2 RISC-V: KVM: Forward SBI DBCN extension to user-space
d9c00f4 KVM: riscv: selftests: Add SBI DBCN extension to get-reg-list test