ARM: dts: msm: specify VADC die temperature channel for smb1355

In SM8150 platforms with SMB1355 configuration, VADC channel
ADC_AMUX_THM2 is used for SMB1355 die temperature.

Expose it in the name of charger_temp.

Change-Id: I32522fcfeed096e09e0cf6ce8cb041d7a8f49737
Signed-off-by: Harry Yang <harryy@codeaurora.org>