dt-bindings: power: qcom,rpmhpd: Add GMXC PD index

Document GMXC (Graphics MXC) power domain index which will be used on
SC8380XP SoCs.

Signed-off-by: Sibi Sankar <quic_sibis@quicinc.com>
Link: https://lore.kernel.org/r/20231025135943.13854-2-quic_sibis@quicinc.com
[Ulf: Re-based to step up the index number]
Signed-off-by: Ulf Hansson <ulf.hansson@linaro.org>

Author: Sibi Sankar

include/dt-bindings/power/qcom,rpmhpd.h

@@ -27,5 +27,6 @@
 #define RPMHPD_DDR              17
 #define RPMHPD_XO               18
 #define RPMHPD_NSP2             19
+#define RPMHPD_GMXC		20
 
 #endif