get platform_data from gpio_keys_button_dev.pdata, and fix a illegal pointer
dereference like this:
[ 51.143776] gpio-keys-polled gpio-keys-polled: missing poll_interval value
[ 51.150852] gpio-keys-polled: probe of gpio-keys-polled failed with error -22
[ 828.159993] gpio-keys-polled gpio-keys-polled: no memory for button data
[ 828.166821] gpio-keys-polled: probe of gpio-keys-polled failed with error -12
Signed-off-by: Furong Xu <xfr@outlook.com>