This commit renames all registers in xtensa/specreg.h to by adding the
prefix XT_REG_. This is done to avoid naming collisions with similar
variable names. A new register file, viz., xt_specreg.h is created. The
previous names are still available to use but have been deprecated.
Closes https://github.com/espressif/esp-idf/issues/12723
Merges https://github.com/espressif/esp-idf/pull/16040
Closes https://github.com/espressif/esp-idf/issues/13629
NMI interrupt level has been freed for all the Xtensa targets, making it possible
for the main application to use it. An example has been added to show how to
proceed.