efi: Use the installed SMBIOS tables

U-Boot should set up the SMBIOS tables during startup, as it does on x86.
Ensure that it does this correctly on non-x86 machines too, by creating
an event spy for last-stage init.

Tidy up the installation-condition code while we are here.

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