smbios: Add more options for the BIOS version string

At present the version string is obtained from PLAIN_VERSION. Some boards
may want to configure this using the device tree, since the build system
can more easily insert things there after U-Boot itself is built. Add this
option to the code.

Also in some cases the version needs to be generated programmatically,
such as when it is stored elsewhere in the ROM and must be read first.
To handle this, keep a pointer around so that it can be updated later.
This works by storing the last string in the context, since it is easier
than passing out a little-used extra parameter.

Provide a function to update the version string.

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