efi: Correct smbios-table installation

At present this code allocates memory when writing the tables and
then unnecessarily adds another memory map when installing it.

Adjust the code to allocate the tables using the normal U-Boot
mechanism. This avoids doing an EFI memory allocation early in
U-Boot, which may use memory that would be overwritten by a
'load' command, for example.

Signed-off-by: Simon Glass <sjg@chromium.org>