net: ipa: move channel type values into "gsi_reg.h"

The gsi_channel_type enumerated type define values used for the
channel type/protocol for event rings and channels.  Move its
definition out of "gsi.c" and into "gsi_reg.h", alongside the
definition of the CH_C_CNTXT_0 register offset and its fields.
Add a comment near the definition of the EV_CH_E_CNTXT_0 register
indicating this type is used for its EV_CHTYPE field.

Signed-off-by: Alex Elder <elder@linaro.org>
Signed-off-by: Jakub Kicinski <kuba@kernel.org>