riscv/purgatory: hard-code obj-y in Makefile

The purgatory/ directory is entirely guarded in arch/riscv/Kbuild.
CONFIG_ARCH_HAS_KEXEC_PURGATORY is bool type.

$(CONFIG_ARCH_HAS_KEXEC_PURGATORY) is always 'y' when Kbuild visits
this Makefile for building.

Signed-off-by: Masahiro Yamada <masahiroy@kernel.org>
Link: https://lore.kernel.org/r/20220625223438.835408-1-masahiroy@kernel.org
Signed-off-by: Palmer Dabbelt <palmer@rivosinc.com>