am335x_evm: Rework board_is_foo() checks

We rework the various board_is_foo() checks to take a pointer to
struct am335x_baseboard_id rather than using a local copy in board.c.
This allows us to make use of the same checks in mux.c as well as fixing
problems when this code could be running from read-only memory.

Reviewed-by: Peter Korsgaard <jacmet@sunsite.dk>
Signed-off-by: Tom Rini <trini@ti.com>