ARMV7: OMAP3: Cleanup extern variables in mem.c

Removed boot_flash_* extern variables.
boot_flash_type was totally unused. The other ones were actually constants, so
they have been replaced with #defines in the board config files.

Signed-off-by: Luca Ceresoli <luca.ceresoli@comelit.it>
Cc: Wolfgang Denk <wd@denx.de>
Cc: Albert Aribaud <albert.aribaud@free.fr>
Cc: Sandeep Paulraj <s-paulraj@ti.com>
Signed-off-by: Sandeep Paulraj <s-paulraj@ti.com>