armv8: Add tlb_allocated to arch global data

When secure ram is used, MMU tables have to be put into secure ram.
To use common MMU code, gd->arch.tlb_addr will be used to host TLB
entry pointer. To save allocated memory for later use, tlb_allocated
variable is added to global data structure.

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