board: ks2: README: Update NAND wording

Traditional KS2 devices supported NAND via the AEMIF peripheral. However,
66AK2G doesn't use the AEMIF but rather the GPMC for NAND. Therefore,
clarify some statements to indicate only certain devices have AEMIF and
in other places just say NAND instead of AEMIF NAND

Signed-off-by: Franklin S Cooper Jr <fcooper@ti.com>
Acked-by: Roger Quadros <rogerq@ti.com>