dts: arc: emsk: Remove emsk_dt.h and CONFIG options from dtsi

There are different interrupt numbers for R22 v R23.  Before this was
being handled via using Kconfig symbol CONFIG_BOARD_EM_STARTERKIT_R23 in
emsk_dt.h.  Since we want to remove use of Kconfig in dts we handle this
via different dtsi files and having the proper one included by the
proper board revision dts file.

Signed-off-by: Kumar Gala <kumar.gala@linaro.org>