refactor(imx8mp): introduce BL2_SIZE and BL31_SIZE

No functional change.

Introduce BLx_SIZE defines and calculate the limits based on the
BLx_BASE and the BLx_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.

Signed-off-by: Marco Felsch <m.felsch@pengutronix.de>
Change-Id: Idae34c1dfcedd35238fe083149080a199d50eed0