ARM: UniPhier: detect the number of flash banks at run-time

Some UniPhier boards are equipped with an expansion slot that
some optional SRAM/NOR-flash cards can be attached to.  So, run-time
detection of the number of flash banks would be more user-friendly.

Until this commit, UniPhier boards have achieved this by (ab)using
board_flash_wp_on() because the boot failed if flash_size got zero.
Fortunately, this problem was solved by commit 70879a92 (flash:
do not fail even if flash_size is zero).

Now it is possible to throw away such a tricky workaround.  This
commit also enables CONFIG_SYS_MAX_FLASH_BANKS_DETECT for further
refactoring.

Signed-off-by: Masahiro Yamada <yamada.m@jp.panasonic.com>