clk: mvebu: add Armada 375 support to the corediv clock driver

This commit adds support for the Core Divider clocks of the Armada
375. Compared to Armada 370 and XP the Core Divider clocks of the 375
cannot be gated: only their ratio can be changed. This is reflected by
the fact that the enable, disable and is_enabled clock operations are
not defined, and that the enable_bit_offset field is also undefined.

Signed-off-by: Thomas Petazzoni <thomas.petazzoni@free-electrons.com>
Signed-off-by: Ezequiel Garcia <ezequiel.garcia@free-electrons.com>
Signed-off-by: Jason Cooper <jason@lakedaemon.net>