mboot64: Fix up clock and UART clock freqs.

The correct clock freq was hiding in the foundation dts, and
It's not 24Mhz, which explains why delays were short.

Still unclear about UART clock - between bootwrapper and xen
I've got two choice, and I chose the bootwrapper value. In
a simulation it doesn't really matter.

Signed-off-by: Andrei Warkentin <andrey.warkentin@gmail.com>