boards: arm: Add Nordic nrf52820 build defines for nrf52833 DK

Add nrf52820 nrfx defines for emulation on the nrf52833 Development Kit.
Fixes assert:
	ASSERTION FAIL [nrf_gpio_pin_present_check(*p_pin)]
	@ WEST_TOPDIR/modules/hal/nordic/nrfx/hal/nrf_gpio.h:525

Signed-off-by: Joakim Andersson <joakim.andersson@nordicsemi.no>