ACPI: Remove CONFIG_GENERATE_ACPI_TABLES

As currently many systems would be barely functional without ACPI,
always generate ACPI tables if supported.

Change-Id: I372dbd03101030c904dab153552a1291f3b63518
Signed-off-by: Vladimir Serbinenko <phcoder@gmail.com>
Reviewed-on: http://review.coreboot.org/4609
Tested-by: build bot (Jenkins)
Reviewed-by: Aaron Durbin <adurbin@google.com>