ARM: shmobile: henninger: specify EXTAL frequency

When creating the initial device tree for the Henninger board,  I've overlooked
that EXTAL frequency needs to be overridden there. The 'sh-sci' driver  managed
to work somehow but the SDHI driver that I've tried to enable just hanged with
the default EXTAL frequency of 0...

Signed-off-by: Sergei Shtylyov <sergei.shtylyov@cogentembedded.com>
Signed-off-by: Simon Horman <horms+renesas@verge.net.au>