boards: nxp: mimxrt1050_evk: fix reference to A0 silicon on EVK

RT1050 EVK rev A boards (with rev A silicon) are not well supported by
Zephyr. Update the documentation page to remove the reference to A0
silicon, as the page describes details specific to the RT1050 EVKB with
rev B silicon.

Signed-off-by: Daniel DeGrasse <daniel.degrasse@nxp.com>