ARM: shmobile: Add r8a7740 Maximum CPU Frequency to DTS

Add 800 MHz to the r8a7740 DTS to describe the maximum CPU frequency.

Signed-off-by: Magnus Damm <damm+renesas@opensource.se>
Tested-by: Geert Uytterhoeven <geert@glider.be>
Signed-off-by: Simon Horman <horms+renesas@verge.net.au>