boards: xtensa: Merge intel_adsp_cavs25 variants into one directory

Merges the intel_adsp_cavs25_tgph and intel_adsp_cavs25 board variants
into a shared directory to reduce code duplication and align with the
layout used by other multicore boards (e.g., lpcxpresso55s69,
mimxrt1170_evk, nrf9160). Note that this change does not affect the
names of the board definitions and therefore does not affect the west
command line parameters needed to build a Zephyr application.

Signed-off-by: Maureen Helm <maureen.helm@intel.com>