doc: update up_squared document to use UEFI bootable method.
doc: update up_squared document to use UEFI bootable method.

for up_squared board, we already support to use UEFI bootable
method to run zephyr tests, so update the document to use
this UEFI method, rather than legacy BIOS stuff.

Signed-off-by: Chen Peng1 <a9104ed43ee1e05962395401b47b879b137cfacf@intel.com>

galak/zephyr,galak/zephyr,galak/zephyr,zephyrproject-rtos/zephyr,zephyrproject-rtos/zephyr,galak/zephyr,zephyrproject-rtos/zephyr,zephyrproject-rtos/zephyr,zephyrproject-rtos/zephyr,galak/zephyr,finikorg/zephyr,finikorg/zephyr,finikorg/zephyr,finikorg/zephyr,finikorg/zephyr