arm64: dts: qcom: msm8916-samsung-e2015: Add touchkey

On the Samsung Galaxy E5 and E7 the touch key is supplied by a single
fixed regulator (enabled via GPIO 97) that supplies both MCU and LED.
Add it to the device tree.

Signed-off-by: Lin, Meng-Bo <linmengbo0689@protonmail.com>
Signed-off-by: Bjorn Andersson <andersson@kernel.org>
Link: https://lore.kernel.org/r/20220724095426.14189-1-linmengbo0689@protonmail.com