arm64: dts: ls1012a: 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.
Accordingly, update the clock-frequency in sysclk to 125M as platform
input clock.

Signed-off-by: Scott Wood <oss@buserror.net>
Signed-off-by: Tang Yuantian <andy.tang@nxp.com>
Signed-off-by: Shawn Guo <shawnguo@kernel.org>