acpi: Add missing RISC-V acpi_table header

The pci_mmc.c driver can generate ACPI info and therefore includes
asm/acpi_table.h. This file does not exist for the RISC-V architecture
and thus code compilation fails when using this driver on RISC-V

Create an empty include file.

Signed-off-by: Heinrich Schuchardt <heinrich.schuchardt@canonical.com>
Reviewed-by: Leo Yu-Chi Liang <ycliang@andestech.com>
Reviewed-by: Rick Chen <rick@andestech.com>