ARM: shmobile: armadillo: dts: Add PWM backlight power supply

Commit 22ceeee1 ("pwm-backlight: Add
power supply support") added a mandatory power supply for the PWM
backlight. Add a fixed 5V regulator and reference it for the backlight
power supply.

Signed-off-by: Laurent Pinchart <laurent.pinchart+renesas@ideasonboard.com>
Signed-off-by: Simon Horman <horms+renesas@verge.net.au>