ARM: shmobile: rcar-gen2: Correct arch timer frequency on RZ/G1E

According to the datasheet, the frequency of the ARM architecture timer
on RZ/G1E depends on the frequency of the ZS clock, just like on R-Car
E2 and V2H.

Signed-off-by: Geert Uytterhoeven <geert+renesas@glider.be>
Signed-off-by: Simon Horman <horms+renesas@verge.net.au>