RISC-V: ACPI: Update the return value of acpi_get_rhct()

RISC-V: ACPI: Update the return value of acpi_get_rhct()

acpi_get_rhct() currently returns pointer to acpi_table_header
structure. But since this is specific to RHCT, return pointer to
acpi_table_rhct structure itself.

Suggested-by: Andrew Jones <ajones@ventanamicro.com>
Signed-off-by: Sunil V L <sunilvl@ventanamicro.com>
Reviewed-by: Andrew Jones <ajones@ventanamicro.com>
Link: https://lore.kernel.org/r/20231018124007.1306159-3-sunilvl@ventanamicro.com
Signed-off-by: Palmer Dabbelt <palmer@rivosinc.com>