mb/ti/beaglebone/board.fmd: Use 'FLASH' as device name

FLASH is often used when accessing FMAP base and size from
fmap_config.h so it's handy to be consistent with all other boards.

Change-Id: Ibba938c72d42ac74dcea8e8e6478ddae510d8c03
Signed-off-by: Arthur Heymans <arthur@aheymans.xyz>
Reviewed-on: https://review.coreboot.org/c/coreboot/+/63349
Tested-by: build bot (Jenkins) <no-reply@coreboot.org>
Reviewed-by: Felix Singer <felixsinger@posteo.net>
Reviewed-by: Angel Pons <th3fanbus@gmail.com>

src/mainboard/ti/beaglebone/board.fmd

@@ -1,4 +1,4 @@
-SDCARD@0x000 32M {
+FLASH@0x000 32M {
     BIOS@0x0 109K {
         BOOTBLOCK@0x0 20K
     }