drivers: counter: mchp_xec: convert to DT_INST defines

Convert driver to use DT_INST_ defines.  The preferred defines for
drivers are DT_INST_.

As part of this change we utilize the device tree for GIRQ info and
rename timer3 to 2 since we are doing this by instance number.

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