[ARM] Make Acorn partition types depend on ACORN_PARTITION

balamurugan reported a problem where it was possible to have
the various Acorn partition types selected in the configuration,
but ACORN_PARTITION disabled.  Since ACORN_PARTITION controls
whether we build fs/partitions/acorn.c, this lead to undefined
references to the adfspart_check_TYPE symbols.

Fix this by making the Acorn partition type symbols depend on
ACORN_PARTITION.

Signed-off-by: Russell King <rmk+kernel@arm.linux.org.uk>