Optimize mmu handling

Previously there was two complete L1 mmu tables where one was only
used when kernel mapping was active and the other when user mapping
was active too. In addition to that there was several sets of L2 mmu
tables which where unused.

Now there's only one complete L1 mmu table shared by all CPUs. There's
one small L1 mmu table for each thread used to keep user mappings. The
small L1 mmu tables takes each 128 bytes and can spans 32 MiB of
virtual memory.

This change saves memory ~32 KiB, but also prepares for multiprocessing
since each thread has its own user mapping.

Due to the change of mmu table handling many low level assembly and C
functions are changed, and some even removed.

Reviewed-by: Etienne Carriere <etienne.carriere@st.com>
Reviewed-by: Pascal Brand <pascal.brand@linaro.org>
Remove some assembly files

Removes kta_table_unpg_asm.S,
Remove tee_pager_unpg_asm.S
Removes tee_mmu_unpg_asm.S
* Replaces assembly implementation of tee_mmu_switch()
  with a C version
* Replaces calls to tee_mmu_invtlb_asid with
  secure_mmu_unifiedtlbinv_byasid

Reviewed-by: Etienne Carriere <etienne.carriere@st.com>
Reviewed-by: Pascal Brand <pascal.brand@linaro.org>
Rename .bss.prebss.* sections to .nozi.*

Renames .bss.prebss.* sections to .nozi.* to be clear that it's
not a "subsection" of .bss and also make the matching in the link
script easier.

plat-vexpress:
* The .nozi section is moved after the .bss section
* The padding added before .nozi by the linker is recorded to
  make it possible to do something useful with the otherwise
  wasted memory

Reviewed-by: Etienne Carriere <etienne.carriere@st.com>
Reviewed-by: Pascal Brand <pascal.brand@linaro.org>
Tested-by: Pascal Brand <pascal.brand@linaro.org> (STM platform)
Tested-by: Jens Wiklander <jens.wiklander@linaro.org> (FVP platform)