smbios: Drop support for SMBIOS2 tables

These tables are a pain since there is no way to handle memory above
4GB. Use SMBIOS3 always.

This should hopefully not create problems on x86 devices, since SMBIOS3
was released seven years ago (2015).

Signed-off-by: Simon Glass <sjg@chromium.org>
Reviewed-by: Heinrich Schuchardt <xypron.glpk@gmx.de>
Reviewed-by: Peter Robinson <pbrobinson@gmail.com>