smbios: Use a struct to keep track of context

At present we pass the ofnode to each function. We also pass the 'eos'
pointer for adding new strings. We don't track the current end of the
string table, so have smbios_string_table_len() to find that.

The code can be made more efficient if it keeps information in a
context struct. This also makes it easier to add more features.

As a first step, switch the ofnode parameter to be a context pointer.
Update smbios_add_prop() at the same time to avoid changing the same
lines of code in consecutive patches.

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