refactor(imx8mn): introduce BL31_SIZE

Introduce BL31_SIZE define and calculate the limit based on the
BL31_BASE and the BL31_SIZE define. Also make use of SZ_128K to make it
easier to read. This is required for later BL31 PIE support since it
drops the calculation based on the BL31_LIMIT and BL31_BASE.

While on it remove the duplicated <lib/utils_def.h> include.

Signed-off-by: Marco Felsch <m.felsch@pengutronix.de>
Change-Id: Ifca40bd5682ef993db986439115abd9e9a66a5b2