board: 7x30: Specify the correct min/max touchscreen coordinates

The userspace interpolates touch coordinates based on max and
min values registered by the driver. These (max and min) values
are derived from the coordinates reported by the touch controller
for a given touchscreen (here the TS connected on 7x30).

Change-Id: Ibc48a0f74473df52205a7135fe779ed96867b10b
Signed-off-by: Anirudh Ghayal <aghayal@codeaurora.org>