ARM: dts: keystone: fix netcp's clocks definitions

The clocks tree for Keystone 2 NTCP devices should be
defined as following:
[refclk] - board dependent
 |- <papllclk> - PLL clock
    |- <paclk13> - fixed factor clock div=3 mul=1
       |- <clkpa> - gated clock
       |- <clkcpgmac> - gated clock
       |- <clksa> - gated clock

Hence, update Keystone 2 DT to follow HW specification.

Signed-off-by: WingMan Kwok <w-kwok2@ti.com>
Signed-off-by: Grygorii Strashko <grygorii.strashko@ti.com>
Signed-off-by: Santosh Shilimkar <santosh.shilimkar@ti.com>