samples: sensor: paj7620: refactor handling of trigger mode

The current sample was always trying to enable trigger mode in the
driver no matter what, causing issues for instances where the sensor
simply has no interrupt pin configured in Devicetree.

Cleaned things up so that enabling trigger mode is done via the
driver's Kconfig option, and cleaned up the Twister testcases
accordingly (plus, made sure they are actually built by setting
min_ram to a value compatibel with the nucleo_f334r8).

README has also been updated to clearly document how to enable
either mode.

Signed-off-by: Benjamin Cabé <benjamin@zephyrproject.org>