ARM: shmobile: sh73a0 dtsi: Add SoC-specific IIC compatible properties

The IIC nodes used the generic compatible properties only.
This causes the driver to fail when using Standard Speed, as the
operational clock is driven by the 104 MHz HP clock:

    i2c-sh_mobile e6820000.i2c: timing values out of range: L/H=0x208/0x1bf
    i2c-sh_mobile: probe of e6820000.i2c failed with error -22

Add the SoC-specific compatible property to fix this.

Signed-off-by: Geert Uytterhoeven <geert+renesas@glider.be>
Reviewed-by: Wolfram Sang <wsa+renesas@sang-engineering.com>
Signed-off-by: Simon Horman <horms+renesas@verge.net.au>