ARM: uniphier: fix recommended board setting in document

The mem_is_flash() in arch/arm/mach-uniphier/micro-support_card.c
writes/reads the tail of each NOR flash bank to check if the device
really resides there.

If CS1_SPLIT were enabled, the support card would always require two
NOR flash devices to be inserted for the correct NOR detection.
This is not probably what we want.

Signed-off-by: Masahiro Yamada <yamada.masahiro@socionext.com>