am335x_evm: am33xx_spl_board_init function and scale core frequency

Add a am33xx_spl_board_init (and enable the PMICs) that we may see,
depending on the board we are running on.  In all cases, we see if we
can rely on the efuse_sma register to tell us the maximum speed.  In the
case of Beaglebone White, we need to make sure we are on AC power, and
are on later than rev A1, and then we can ramp up to the PG1.0 maximum
of 720Mhz.  In the case of Beaglebone Black, we are either on PG2.0 that
supports 1GHz or PG2.1.  As PG2.0 may or may not have efuse_sma set, we
cannot rely on this probe.  In the case of the GP EVM, EVM SK and IDK we
need to rely on the efuse_sma if we are on PG2.1, and the defaults for
PG1.0/2.0.

Signed-off-by: Tom Rini <trini@ti.com>