mtd: nand: Confine MTD_NAND_SH_FLCTL to SUPERH

As of commit a521422e ("ARM: shmobile: mackerel: Remove Legacy C
board code"), the Renesas SuperH FLCTL driver is no longer used on ARM
SH-Mobile SoCs. Restrict the dependencies, unless compile-testing.

Signed-off-by: Geert Uytterhoeven <geert+renesas@glider.be>
Signed-off-by: Brian Norris <computersforpeace@gmail.com>