defconfig: apq8084: Enable virtual key driver on 8084

There are four virtual buttons on touch panel of 8084 CDP/MTP. The
virtual key driver needs to be enabled for users to use these buttons.

Change-Id: I2c52ce0c533b7907b626eed0068efc7ba0dc27b8
Signed-off-by: Chun Zhang <chunz@codeaurora.org>