omap3evm: Fix mechanism to identify board revision

Function omap3_evm_get_revision() - to identify the
board revision was called at end of setup_net_chip().

Board revision can be ascertained only by identifying
the Ethernet chipset - but combining setup operations
with revision detection isn't a good idea. So, moved
the function after call to setup_net_chip().

Function setup_net_chip() should be ideally be called
only when CONFIG_CMD_NET is defined. But this leaves
the board revision "undetected". This patch allows
static definition of revision or default fallback to
the latest revision.

Signed-off-by: Sanjeev Premi <premi@ti.com>
Signed-off-by: Sandeep Paulraj <s-paulraj@ti.com>