arm64: fix incorrect memory region size in TCR_EL2

arm64: fix incorrect memory region size in TCR_EL2

The maximum and minimum values for TxSZ depend on level of
translation as per AArch64 Virtual Memory System Architecture.
According to ARM specification DDI0487A_h (sec D4.2.2, page 1752),
the minimum TxSZ value is 16. If TxSZ is programmed to a value
smaller than 16 then it is IMPLEMENTATION DEFINED.

This patch sets T0SZ to (64-48)bits since XEN uses all 4 levels
to cover 48bit (256TB) virtual address instead of value zero.

Signed-off-by: Shanker Donthineni <shankerd@codeaurora.org>
Acked-by: Julien Grall <julien.grall@arm.com>