armv8: ls2080a: Drop early MMU for SPL build

Early MMU improves performance especially on emulators. However, the
early MMU is left enabled after the first stage of SPL boot. Instead
of flushing D-cache and dealing with re-enabling MMU for the second
stage U-Boot, disabling it for SPL build simplifies the process. The
performance penalty is unnoticeable on the real hardware. As of now,
SPL boot is not supported by existing emulators. So this should have
no impact on emulators.

Signed-off-by: York Sun <york.sun@nxp.com>