xtensa: gdbstub: support more registers

Stack offset for spilled registers can be larger than 256 bytes, e.g.
on ADSP platforms with additional floating point and SIMD registers.
Register indices can also be larger than 255. Update both types to
support larger values.

Signed-off-by: Guennadi Liakhovetski <guennadi.liakhovetski@linux.intel.com>