The initial value of the interrupt mask register may be different from
the H/W manual at the startup of the kernel by setting from the
bootloader. Since the error interrupts may be unmasked, the driver sets
initial value.

The initial value is only known for R-Car Gen2 and Gen3 platforms so
limit the initialization to those platforms.

Based on work from Masaharu Hayakawa.
Signed-off-by: Niklas Söderlund <niklas.soderlund+renesas@ragnatech.se>
Tested-by: Wolfram Sang <wsa+renesas@sang-engineering.com>
Reviewed-by: Wolfram Sang <wsa+renesas@sang-engineering.com>
Reviewed-by: Simon Horman <horms+renesas@verge.net.au>
Signed-off-by: Ulf Hansson <ulf.hansson@linaro.org>