ARM64: dts: bcm2710-rpi-3-b: Adds buttons for speaker profile

ARM64: dts: bcm2710-rpi-3-b: Adds buttons for speaker profile

Tizen Speaker Profile uses a keypad that consists of 5 buttons for
up/down/left/right/center. The buttons are connected to GPIO pins as
follows.

GPIO16 : KEY_VOLUMEUP
GPIO13 : KEY_VOLUMEDOWN
GPIO6 : KEY_FASTFORWARD
GPIO12 : KEY_REWIND
GPIO26 : KEY_PLAYPAUSE

gpio=6,12,13,16,26=ip,pu at config.txt means that 6,12,13,16,26 pins are used as
input and pullup mode.

Change-Id: I8b7fe4fa360c2d17042e31270702d7abe49a5977
Signed-off-by: Jaechul Lee <jcsing.lee@samsung.com>