Add generation of the following macros:
DT_N_<node-id>_REG_IDX_<idx>_EXISTS 1
DT_N_<node-id>_IRQ_IDX_<idx>_EXISTS 1
This will allow us to use IS_ENABLED() in DT_REG_HAS_IDX and
DT_IRQ_HAS_IDX which matches behavior of other DT_*_HAS_* macros as
well as lets use these with COND_CODE_1.
Signed-off-by: Kumar Gala <kumar.gala@linaro.org>