ARM: keystone2: K2G: Add support for different arm/device speeds

The maximum device and arm speeds can be determined by reading
EFUSE_BOOTROM register. As there is already a framework for reading this
register, adding support for all possible speeds on k2g devices.

Signed-off-by: Lokesh Vutla <lokeshvutla@ti.com>
Signed-off-by: Nishanth Menon <nm@ti.com>
Reviewed-by: Tom Rini <trini@konsulko.com>