ARM: shmobile: Update r8a7791 CPU freq to 1500MHz in DTS

The correct maximum CPU frequency for r8a7791 is 1500 MHz
so update the r8a7791 SoC DTS to reflect this.

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