ARM: shmobile: r8a7791 clock: add QSPI clocks

The QSPI clock divider value depends on the MD1, MD2, and MD3 mode
switches.

Signed-off-by: Geert Uytterhoeven <geert+renesas@linux-m68k.org>
Acked-by: Laurent Pinchart <laurent.pinchart@ideasonboard.com>
Acked-by: Magnus Damm <damm@opensource.se>
Signed-off-by: Simon Horman <horms+renesas@verge.net.au>