arch: xtensa: Refine HiFi sharing Kconfigs

Splits HiFi sharing into two different models.
 1. XTENSA_EAGER_HIFI_SHARING - unconditional save/restore
    of the HiFi registers when context switching
 2. XTENSA_LAZY_HIFI_SHARING - on demand save/restore of
    of the HiFi registers. If a thread does not use the
    HiFi registers, they are neither saved nor restored.

To maintain backwards compatibility, the eager model is the
default model when XTENSA_HIFI_SHARING is enabled.

Signed-off-by: Peter Mitsis <peter.mitsis@intel.com>