clock_am43xx:Set the MAC clock to /5 for OPP100

When EMAC is in the boot order, the boot ROM sets OPP50 and the
MAC clock is set to /2. SPL needs to change it to /5 for Ethernet
to generate the correct txclk. This patch sets it correctly.

Signed-off-by: Steve Kipisz <s-kipisz2@ti.com>