hw/riscv/virt-acpi-build.c: Update FADT and MADT versions

hw/riscv/virt-acpi-build.c: Update FADT and MADT versions

RISC-V support is added only in ACPI 6.6. According to the ACPI 6.6
specification, the minor version of the Fixed ACPI Description Table
(FADT) should be 6, and the Multiple APIC Description Table (MADT)
should use revision 7. So, update the RISC-V FADT and MADT to reflect
correct versions.

Update the code comments to reflect ACPI 6.6 version details.

Signed-off-by: Sunil V L <sunilvl@ventanamicro.com>
Acked-by: Michael S. Tsirkin <mst@redhat.com>
Reviewed-by: Daniel Henrique Barboza <dbarboza@ventanamicro.com>
Reviewed-by: Nutty Liu <liujingqi@lanxincomputing.com>
Message-ID: <20250724110350.452828-3-sunilvl@ventanamicro.com>
Signed-off-by: Alistair Francis <alistair.francis@wdc.com>