riscv: acpi: update FADT revision check to 6.6

riscv: acpi: update FADT revision check to 6.6

ACPI 6.6 is required for RISC-V as it introduces RISC-V specific
tables such as RHCT (RISC-V Hart Capabilities Table) and
RIMT (RISC-V I/O Mapping Table).

Update the FADT revision check from 6.5 to 6.6 and remove
the TODO comment since ACPI 6.6 has been officially released.

Signed-off-by: Yufeng Wang <wangyufeng@kylinos.cn>
Reviewed-by: Sunil V L <sunilvl@oss.qualcomm.com>
Acked-by: Heinrich Schuchardt <heinrich.schuchardt@canonical.com>
Reviewed-by: Yao Zi <me@ziyao.cc>
Link: https://patch.msgid.link/20260305091433.83983-1-r4o5m6e8o@163.com
Signed-off-by: Paul Walmsley <pjw@kernel.org>