Use the general touchscreen method to config the max pressure for
touch tsc2046(data sheet suggest 8 bit pressure), otherwise, for
ABS_PRESSURE, when config the same max and min value, weston will
meet the following issue,

[17:19:39.183] event1  - ADS7846 Touchscreen: is tagged by udev as: Touchscreen
[17:19:39.183] event1  - ADS7846 Touchscreen: kernel bug: device has min == max on ABS_PRESSURE
[17:19:39.183] event1  - ADS7846 Touchscreen: was rejected
[17:19:39.183] event1  - not using input device '/dev/input/event1'

This will then cause the APP weston-touch-calibrator can't list touch devices.

root@imx6ul7d:~# weston-touch-calibrator
could not load cursor 'dnd-move'
could not load cursor 'dnd-copy'
could not load cursor 'dnd-none'
No devices listed.

And accroding to binding Doc, "ti,x-max", "ti,y-max", "ti,pressure-max"
belong to the deprecated properties, so remove them. Also for "ti,x-min",
"ti,y-min", "ti,x-plate-ohms", the value set in dts equal to the default
value in driver, so are redundant, also remove here.
Signed-off-by: Haibo Chen <haibo.chen@nxp.com>
Signed-off-by: Shawn Guo <shawnguo@kernel.org>