ARM: dts: am335x-evm: fix comments for lcd pins

In the comments, LCD pins 16-23 were numbered in the wrong order.
Fix this and use proper pinmux constants for all entries while we
are at it.

Signed-off-by: Wolfram Sang <wsa@sang-engineering.com>
Cc: Benoit Parrot <bparrot@ti.com>
Tested-by: Darren Etheridge <detheridge@ti.com>
[tony@atomide.com: updated description]
Signed-off-by: Tony Lindgren <tony@atomide.com>