arm64: zynqmp: Setup default si570 frequency to 156.25MHz

arm64: zynqmp: Setup default si570 frequency to 156.25MHz

All si570 mgt chips have factory default 156.25MHz but DT changed it to
148.5MHz. After tracking it is pretty much c&p fault taken from Zynq
zc702/zc706 boards where 148.5MHz was setup as default because it was
requirement for AD7511 chip available on these boards.
ZynqMP board don't contain this chip that's why factory default frequency
can be used.

Link: https://lore.kernel.org/r/65a53776cbc5e4586f58da57a4b99e4d5c6c26a7.1704728353.git.michal.simek@amd.com
Signed-off-by: Michal Simek <michal.simek@amd.com>