ARM: dts: imx53-qsb: Improve the parallel display description

MX53_PAD_GPIO_1__PWM2_PWMO controls the backlight of the parallel
Seiko display and MX53_PAD_EIM_D24__GPIO3_24 controls the display power.

Reflect that in the devicetree for better description of the board.

Without these entries there is no LCD output on the Seiko display, unless
the bootloader has previously configured these pins.

Signed-off-by: Fabio Estevam <festevam@denx.de>
Signed-off-by: Shawn Guo <shawnguo@kernel.org>