dt-bindings: qoriq-clock: Add coreclk

ls1012a has separate input root clocks for core PLLs versus the platform
PLL, with the latter described as sysclk in the hw docs.
Update the qoriq-clock binding to allow a second input clock, named
"coreclk".  If present, this clock will be used for the core PLLs.

Signed-off-by: Scott Wood <oss@buserror.net>
Signed-off-by: Tang Yuantian <andy.tang@nxp.com>
Acked-by: Rob Herring <robh@kernel.org>
Signed-off-by: Stephen Boyd <sboyd@codeaurora.org>