protobuf: added description of messages to store the AArch64 CPU state

These messages are constructed in the same spirit as ARM and x86 ones
except for two major points:

* general-purpose registers are stored in a variable-length array
  of uint64's: the architecture provides 32 general-purpose registers
  that makes it unfeasible to create a separate protobuf field
  for each of them since it requires a lot of "copy-paste" to convert
  between the struct pt_regs and protobuf message; the length of
  the array storing registers is to be checked by the architecture-
  dependent CRIU code;

* AArch64 FP/SIMD registers are 128 bit long while protobuf lacks
  the support for integers of this size; the FP/SIMD registers
  are stored in an array of uint64, two consecutive elements
  of the array represent a single FP/SIMD register.

Signed-off-by: Alexander Kartashov <alekskartashov@parallels.com>
Signed-off-by: Christopher Covington <cov@codeaurora.org>
Signed-off-by: Pavel Emelyanov <xemul@parallels.com>