ARM: shmobile: r8a7793: Add QSPI device to DT

Instantiate the QSPI controller in the r8a7793 device tree.

Based on similar work for the r8a7794 by Hisashi Nakamura and
Sergei Shtylyov.

Signed-off-by: Simon Horman <horms+renesas@verge.net.au>