davinci: board-dm646x-evm.c: arrange related code together

Currently all the #defines and static variables in the
board-dm646x-evm.c file are located right at the start
of the file because of which the related code is not
together - making reading the code difficult.

This patch moves around the code keeping related code
together.

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