platform: fpga: Remove redundant platform specific features

Any platform feature that belongs to a hart, have already been moved to
hart features and are detected at run time. The remaining platform features
are identical to platform default features.

Use the platform default features instead of defining a separate copy of it.

Signed-off-by: Atish Patra <atish.patra@wdc.com>
Tested-by: Jonathan Balkind <jbalkind@cs.princeton.edu>
Reviewed-by: Anup Patel <anup.patel@wdc.com>