ARM: dts: zero-w: Better names for GPIOs 14 & 15

GPIOs 14 and 15 are currently labelled TXD0 and RXD0, which matches
comments on the schematic but 1) doesn't reflect their likely usage
(i.e. UART1) and 2) clashes with GPIOs 32 and 33. Adopt the common
naming for those two pins on BT-equipped Pis - TXD1 and RXD1.

See: https://github.com/raspberrypi/linux/issues/5058



Signed-off-by: Phil Elwell <phil@raspberrypi.com>