tests: kernel: context: Convert to new dts macros

Set the TICK_IRQ on litex and rv32m1 based on DT_IRQN().  For litex we
use DT_NODELABEL(timer0) and on rv32m1 we use DT_ALIAS(system_lptmr) to
determine the timer device.

Signed-off-by: Kumar Gala <kumar.gala@linaro.org>