ARM: fix CONFIG_SPL_MAX_SIZE semantics

Remove SPL-related ASSERT() in arch/arm/cpu/u-boot.lds
as this file is never used for SPL builds.

Rewrite the ASSERT() in arch/arm/cpu/u-boot-spl.lds
to separately test image (text,data,rodata...) size,
BSS size, and full footprint each against its own max,
and make Tegra boards check full footprint.

Also, output section mmutable is not used in SPL builds.
Remove it.

Finally, update README regarding the (now homogeneous)
semantics of CONFIG_SPL_[BSS_]MAX_SIZE and add the new
CONFIG_SPL_MAX_FOOTPRINT macro.

Signed-off-by: Albert ARIBAUD <albert.u.boot@aribaud.net>
Reported-by: Benoît Thébaudeau <benoit.thebaudeau@advansee.com>