soc: renesas: rcar-rst: Add support for R-Car E3

Add support for R-Car E3 (R8A77990) to the R-Car RST driver.
This driver is needed for the clock driver to work.

Signed-off-by: Takeshi Kihara <takeshi.kihara.df@renesas.com>
[shimoda: rebase]
Signed-off-by: Yoshihiro Shimoda <yoshihiro.shimoda.uh@renesas.com>
Reviewed-by: Geert Uytterhoeven <geert+renesas@glider.be>
Signed-off-by: Simon Horman <horms+renesas@verge.net.au>