ARM: nxp_imx: rt10xx: get systick freq from 'sysclk' DT nodelabel

So far systick frequency was duplicated in 'sysclk' DT nodelabel's
'clock-frequency' property and in per SoC SYS_CLOCK_HW_CYCLES_PER_SEC
default value. Set the latter based on DT configuration.

Signed-off-by: Marcin Niestroj <m.niestroj@emb.dev>