Documentation: Fix historical inconsistency in STMPE DT doc

Previously a generic binding 'i2c-client-wake' was created which
enabled I2C devices to register themselves as wake-up devices.
This binding was later over-thrown by 'wakeup-source'. The STMPE
driver was fixed-up, but the document was neglected. This patch
aims to rectify that.

Cc: Samuel Ortiz <sameo@linux.intel.com>
Cc: devicetree-discuss@lists.ozlabs.org
Signed-off-by: Lee Jones <lee.jones@linaro.org>
Signed-off-by: Samuel Ortiz <sameo@linux.intel.com>