ARM: shmobile: r8a7793: remove deprecated #gpio-range-cells

Commit a1bc260b ("gpio: clean up gpio-ranges documentation")
declares the above property deprecated. That was more than 2 years ago.
Remove it, so it doesn't get copied around needlessly.

Based on similar work for the r8a7791 and r8a7794 by Wolfram Sang.

Cc: Wolfram Sang <wsa+renesas@sang-engineering.com>
Signed-off-by: Simon Horman <horms+renesas@verge.net.au>
Reported-by: Wolfram Sang <wsa+renesas@sang-engineering.com>
Acked-by: Wolfram Sang <wsa+renesas@sang-engineering.com>