ACPI: SPCR: Add support for DBG2 RISC-V SBI port subtype

ACPI: SPCR: Add support for DBG2 RISC-V SBI port subtype

Commit 4aca2bef90bd1296 ("ACPICA: Headers: Add RISC-V SBI Subtype to
DBG2") added the definition of ACPI_DBG2_RISCV_SBI_CON.

Continue to implement its function so that the parameters of UART can
be configured correctly.

Subsequent calls to setup_earlycon() will reuse the earlycon based on
SBI.

Signed-off-by: Chen Pei <cp0613@linux.alibaba.com>
Reviewed-by: Guo Ren (Alibaba Damo Academy) <guoren@kernel.org>
Link: https://patch.msgid.link/20250910094136.4423-1-cp0613@linux.alibaba.com
[ rjw: Changelog edits ]
Signed-off-by: Rafael J. Wysocki <rafael.j.wysocki@intel.com>