k64: Move uart console pins to board pinmux tables

Moves the uart console pins from the k64 soc init to the frdm_k64f and
hexiwear_k64 board pinmux tables. Not having these pins in the board
pinmux tables led one to believe that no pins in PORTB were being used
on the hexiwear_k64 board, and thus the port was incorrectly disabled by
default.

Also fixes PORTB to be enabled by default if the uart console is used.

Change-Id: Ide6b7b34dfba8a75a02a8f2bf37cce843afb92f1
Signed-off-by: Maureen Helm <maureen.helm@nxp.com>