The Cortex-A53 ports are generic and can be used as a starting point
for other Armv8-A application processors. Therefore, rename
`ARM_CA53_64_BIT` to `Arm_AARCH64` and `ARM_CA53_64_BIT_SRE` to
`Arm_AARCH64_SRE`.

With this renaming, existing projects that use old port, should
migrate to renamed port as follows:

* `ARM_CA53_64_BIT` -> `Arm_AARCH64`
* `ARM_CA53_64_BIT_SRE` -> `Arm_AARCH64_SRE`

Signed-off-by: Devaraj Ranganna <[email protected]>