fix: 'sp_mk_generator.py' reference to undef var

The script 'sp_mk_generator.py' was reworked in [1]. There was a
reference the variable 'data' left. This variable 'data' used to refer
to the json data of a the sp layout file.
This patch fixed the reference with the proper variable according to the
rework [1].

[1] https://git.trustedfirmware.org/TF-A/trusted-firmware-a.git/commit/?id=a96a07bfb66b7d38fe3da824e8ba183967659008



Signed-off-by: J-Alves <joao.alves@arm.com>
Change-Id: I9ddbfa8d55a114bcef6997920522571e070fc7d2