boards: riscv: rv32m1_vega: fix PWM period and flags

The PWM driven LEDs had a wrong period: 0 nsec. A value of 20 msec has
been chosen as most other boards do for PWM driven LEDs. The polarity
has been set to normal (LEDs are driven active high).

Signed-off-by: Gerard Marull-Paretas <gerard.marull@nordicsemi.no>