target-arm: Implement AArch64 TTBR*

target-arm: Implement AArch64 TTBR*

Implement the AArch64 TTBR* registers. For v7 these were already 64 bits
to handle LPAE, but implemented as two separate uint32_t fields.
Combine them into a single uint64_t which can be used for all purposes.
Since this requires touching every use, take the opportunity to rename
the field to the architectural name.

Signed-off-by: Peter Maydell <peter.maydell@linaro.org>
Reviewed-by: Peter Crosthwaite <peter.crosthwaite@xilinx.com>