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