drivers: ipm: fix build issue from DT macro conversion

Since idc_read/idc_write are used outside of the driver we can't use the
DT_INST_... form (that utilize DT_DRV_COMPAT) of the macro's in
ipm_cavs_idc_priv.h.  Use DT_INST(0, intel_cavs_idc) explicitly in the
header to fix the build issue.

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