ARM: dts: msm: Add wake-gpio for qcs405-iot-sku6 variant only

GPIO 21 can be used for either PCIe or mute button. Hence add
wake-gpio property only for qcs405-iot-sku6 variant only so that
mute button works for other varaints. Also delete mute button node
for allowing PCIe EP to wakeup PCIe RC.

Change-Id: Ic3a327b855d50ddac0b8c0efdb24a1742300108f
Signed-off-by: Vijayavardhan Vennapusa <vvreddy@codeaurora.org>