boards/x86: up_squared: fix UART interrupt triggers
boards/x86: up_squared: fix UART interrupt triggers

The triggers for UART interrupts are actually active-low.

Signed-off-by: Daniel Leung <2aac00f4539b3e5f57432b95eb7a2282893c76e6@gmail.com>

galak/zephyr,nashif/zephyr,GiulianoFranchetto/zephyr,finikorg/zephyr,punitvara/zephyr,zephyrproject-rtos/zephyr,Vudentz/zephyr,galak/zephyr,explora26/zephyr,Vudentz/zephyr,nashif/zephyr,galak/zephyr,GiulianoFranchetto/zephyr,kraj/zephyr,punitvara/zephyr,GiulianoFranchetto/zephyr,nashif/zephyr,ldts/zephyr,Vudentz/zephyr,kraj/zephyr,ldts/zephyr,zephyrproject-rtos/zephyr,nashif/zephyr,GiulianoFranchetto/zephyr,punitvara/zephyr,zephyrproject-rtos/zephyr,Vudentz/zephyr,punitvara/zephyr,ldts/zephyr,zephyrproject-rtos/zephyr,finikorg/zephyr,zephyrproject-rtos/zephyr,ldts/zephyr,finikorg/zephyr,Vudentz/zephyr,punitvara/zephyr,ldts/zephyr,kraj/zephyr,Vudentz/zephyr,explora26/zephyr,explora26/zephyr,explora26/zephyr,finikorg/zephyr,explora26/zephyr,GiulianoFranchetto/zephyr,kraj/zephyr,galak/zephyr,galak/zephyr,finikorg/zephyr,nashif/zephyr,kraj/zephyr