pinctrl: meson: meson8b: rename the NAND DQS pin definitions

The NAND DQS pins are currently named nand_dqs_0 and nand_dqs_1.
However, they both seem to have the same function, just exposed on
different pins (unlike the ethernet TX pins for example, where there's
eth_txd0..3 - all of these can be active at the same time as they are
different data lines).
Rename the NAND DQS pins to nand_dqs_15 and nand_dqs_18 to reflect that
it's the same functionality just exposed on different pins (BOOT_15 and
BOOT_18).

Signed-off-by: Martin Blumenstingl <martin.blumenstingl@googlemail.com>
Acked-by: Kevin Hilman <khilman@baylibre.com>
Signed-off-by: Linus Walleij <linus.walleij@linaro.org>