armv8/cache.S: Read sysreg fields through ubfx

Improve the file's readability and conciseness by using the appropriate
Aarch64 instruction: ubfx (unsigned bitfield extract). This makes the
code easier to follow as it directly manipulates the offsets and widths
of the fields read from system registers, as they are expressed in the
Standard (ARM ARM). This has the added benefit (albeit arguably
negligible) of reducing the final code size.

Signed-off-by: Pierre-Clément Tosi <ptosi@google.com>