arm: omap: board-am3517: use generic dpi panel's gpio handling

The am3517 board file currently requests gpios required to configure the sharp
lq DPI panel, and provides platform_enable/disable callbacks to configure them.

These tasks have been moved to the generic dpi panel driver itself and shouldn't
be done in the board files.

Remove the gpio requests and the platform callbacks from the board file.
Add the gpio information to generic dpi panel's platform data so that it's
passed to the panel driver.

Note: It's not clear why the GPIOs were muxed as input signals in PULL down mode
in am3517_evm_display_init(). Also, only the LCD_PANEL_PWR was toggled in the
platform_enable/disable calls, the generic DPI panel driver will now toggle all
the three gpios on panel's disable/enable. We need to test if these changes to
see if they have any impact or not.

Signed-off-by: Archit Taneja <archit@ti.com>
Cc: Tony Lindgren <tony@atomide.com>
Cc: Vaibhav Hiremath <hvaibhav@ti.com>