ARM: keystone2: Add support for getting external clock dynamically

One some keystone2 platforms like K2G ICE, there is an option
to switch between 24MHz or 25MHz as sysclk. But the existing
driver assumes it is always 24MHz. Add support for getting
all reference clocks dynamically by reading boot pins.

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