ARM: uniphier: refactor Support Card init code

Splitting reset assertion (support_card_reset) and deassertion
(support_card_init) is not adding much value any more.  Handle
all the initialization of Support Card in support_card_init(),
then remove support_card_reset().

Also, detect_num_flash_banks() can have a static qualifier.

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