smbios: Drop the eos parameter

We can store this in the context and avoid passing it to each function.
This makes it easier to follow and will also allow keeping track of the
end of the string table (in future patches).

Add an 'eos' field to the context and create a function to set it up.

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