omap3: Drop CONFIG_OMAP3_EVM, switch to CONFIG_TARGET_OMAP3_EVM when needed

We make use of CONFIG_OMAP3_EVM today to know when to do a specific
tweak in MUSB.  This can be tested on via CONFIG_TARGET_OMAP3_EVM
instead, so switch there so we can drop the now unused symbol
CONFIG_OMAP3_EVM.  In investigating what to do about the symbol usage we
see that the cairo board defines the same function, but never called it
(as it does not define CONFIG_OMAP3_EVM) and was just returning anyhow,
so drop that function from that board.

Cc: "Albert ARIBAUD (3ADEV)" <albert.aribaud@3adev.fr>
Cc: Marek Vasut <marex@denx.de>
Signed-off-by: Tom Rini <trini@konsulko.com>