ARM: dts: rainier: Configure ball Y23 as GPIOP7 for MCLR_VPP

GPIOP7 is used in the Rainier design to manage the state of a
microcontroller elsewhere in the system but its ball, Y23, is the
driver of the heartbeat LED on the ast2600-evb and the SoC defaults Y23
at power-on to the pulse-train behaviour used to drive the LED. This
causes much confusion for the micro in the Rainier system, so hog the
line as early as possible.

Signed-off-by: Andrew Jeffery <andrew@aj.id.au>
Signed-off-by: Joel Stanley <joel@jms.id.au>