ARM: shmobile: rcar-gen2: Obtain extal frequency from DT

On some R-Car Gen2 SoCs, the frequency of the ARM architecture timer
depends on the frequency of the external clock crystal.  Currently the
latter is determined indirectly from the state of the mode pins, which
is a relic predating DT.

Obtain the external clock crystal frequency from DT instead, removing
the dependency on the mode pins.

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