i2c: rcar_i2c: Enable configuring SCL rise and fall times

The Linux i2c driver supports i2c-scl-rising-time-ns,
and i2c-scl-falling-time-ns, but U-Boot uses hard-coded values
for these values.

Update the calculation by fetching them from the device tree if
present and use the previous values as the default if they are
missing.

Signed-off-by: Adam Ford <aford173@gmail.com>
Reviewed-by: Heiko Schocher <hs@denx.de>