ARM: OMAP: omap3evm: fix new sparse warning

ARM: OMAP: omap3evm: fix new sparse warning

Commit e54adb1e79762d3591459e5b0e9b9ff578e33fdb ("ARM: OMAP: omap3evm:
cleanup revision bits") adds a new sparse[1] warning:

arch/arm/mach-omap2/board-omap3evm.c:90:4: warning: symbol 'get_omap3_evm_rev' was not declared. Should it be static?

This symbol no longer appears to be used outside this file, so mark it
static and remove the export.

...

1. https://sparse.wiki.kernel.org/index.php/Main_Page

Signed-off-by: Paul Walmsley <paul@pwsan.com>
Cc: Igor Grinberg <grinberg@compulab.co.il>
Cc: Tony Lindgren <tony@atomide.com>