arm64: dts: renesas: spider-cpu: Add GP LEDs

Describe the two General Purpose LEDs LED7 and LED8 on the Spider CPU
board, so they can be used as indicator LEDs.

Note that General Purpose LEDs LED9 to LED11 are not added, as they are
connected to GPIO block 4, which can only be accessed from the Control
Domain.

Signed-off-by: Geert Uytterhoeven <geert+renesas@glider.be>
Tested-by: Kieran Bingham <kieran.bingham+renesas@ideasonboard.com>
Reviewed-by: Kieran Bingham <kieran.bingham+renesas@ideasonboard.com>
Reviewed-by: Yoshihiro Shimoda <yoshihiro.shimoda.uh@renesas.com>
Tested-by: Yoshihiro Shimoda <yoshihiro.shimoda.uh@renesas.com>
Link: https://lore.kernel.org/r/fdaf6c700b624851039a60733c7f73a413c6d2c5.1690447094.git.geert+renesas@glider.be