cmd: provide command sbi

Provide a command to display information about the SBI implementation.

The output might look like:

=> sbi
SBI 0.2
OpenSBI
Extensions:
  sbi_set_timer
  sbi_console_putchar
  sbi_console_getchar
  sbi_clear_ipi
  sbi_send_ipi
  sbi_remote_fence_i
  sbi_remote_sfence_vma
  sbi_remote_sfence_vma_asid
  sbi_shutdown
  SBI Base Functionality
  Timer Extension
  IPI Extension
  RFENCE Extension
  Hart State Management Extension

The command can be used to construct a unit test checking that the
communication with the SEE is working.

Signed-off-by: Heinrich Schuchardt <xypron.glpk@gmx.de>
Reviewed-by: Atish Patra <atish.patra@wdc.com>
Reviewed-by: Bin Meng <bin.meng@windriver.com>
Tested-by: Bin Meng <bin.meng@windriver.com>
Reviewed-by: Pragnesh Patel <pragnesh.patel@openfive.com>
Tested-by: Pragnesh Patel <pragnesh.patel@openfive.com>
Reviewed-by: Rick Chen <rick@andestech.com>
Tested-by: Rick Chen <rick@andestech.com>