ARM: shmobile: r8a7779: Add Maximum CPU Frequency to DTS

Add 1GHz to the r8a7779 DTS to describe the maximum CPU frequency.

Based on work by Magnus dam for the r8a7740 SoC.

Cc: Magnus Damm <damm+renesas@opensource.se>
Signed-off-by: Simon Horman <horms+renesas@verge.net.au>