clk: renesas: r8a77990: Add Z2 clock

Adds support for R-Car E3 (r8a77990) Z2 clock.

Signed-off-by: Takeshi Kihara <takeshi.kihara.df@renesas.com>
[simon: reworked changelog; rebased]
Signed-off-by: Simon Horman <horms+renesas@verge.net.au>
Signed-off-by: Geert Uytterhoeven <geert+renesas@glider.be>