platform: Enable all drivers by default.

The drivers and libfdt are built as libsbiutils.a
instead of libplatsbi.a.

libsbiutils.a are not built per platform specific. Thus,
enable all drivers by default.

Signed-off-by: Atish Patra <atish.patra@wdc.com>
Acked-by: Anup Patel <anup.patel@wdc.com>