x86: coreboot: Collect the address of the ACPI tables

At present any ACPI tables created by prior-stage firmware are ignored.
It is useful to be able to view these in U-Boot.

Pick this up from the sysinfo tables and display it with the cbsysinfo
command. This allows the 'acpi list' command to work when booting from
coreboot.

Adjust the global_data condition so that acpi_start is available even if
table-generation is disabled.

Signed-off-by: Simon Glass <sjg@chromium.org>
Reviewed-by: Bin Meng <bmeng.cn@gmail.com>
Tested-by: Bin Meng <bmeng.cn@gmail.com>