davinci: fix section mismatch warning in arch/arm/mach-davinci/board-dm646x-evm.c

A section mismatch is reported for gpio_led_platform_data
as it is referenced by a non annotated function (evm_led_setup)

This patch fixes the issue by converting the __initconst to const
as is the case in arch/arm/mach-davinci/board-dm644x-evm.c file.

Signed-off-by: Sekhar Nori <nsekhar@ti.com>
Signed-off-by: Kevin Hilman <khilman@deeprootsystems.com>