The RTAS work area allocator uses code that is built by
GENERIC_ALLOCATOR, so the PSERIES Kconfig should select the
required Kconfig symbol to fix multiple build errors.

powerpc64-linux-ld: arch/powerpc/platforms/pseries/rtas-work-area.o: in function `.rtas_work_area_allocator_init':
rtas-work-area.c:(.init.text+0x288): undefined reference to `.gen_pool_create'
powerpc64-linux-ld: rtas-work-area.c:(.init.text+0x2dc): undefined reference to `.gen_pool_set_algo'
powerpc64-linux-ld: rtas-work-area.c:(.init.text+0x310): undefined reference to `.gen_pool_add_owner'
powerpc64-linux-ld: rtas-work-area.c:(.init.text+0x43c): undefined reference to `.gen_pool_destroy'
powerpc64-linux-ld: arch/powerpc/platforms/pseries/rtas-work-area.o:(.toc+0x0): undefined reference to `gen_pool_first_fit_order_align'
powerpc64-linux-ld: arch/powerpc/platforms/pseries/rtas-work-area.o: in function `.__rtas_work_area_alloc':
rtas-work-area.c:(.ref.text+0x14c): undefined reference to `.gen_pool_alloc_algo_owner'
powerpc64-linux-ld: rtas-work-area.c:(.ref.text+0x238): undefined reference to `.gen_pool_alloc_algo_owner'
powerpc64-linux-ld: arch/powerpc/platforms/pseries/rtas-work-area.o: in function `.rtas_work_area_free':
rtas-work-area.c:(.ref.text+0x44c): undefined reference to `.gen_pool_free_owner'

Fixes: 43033bc6 ("powerpc/pseries: add RTAS work area allocator")
Signed-off-by: Randy Dunlap <rdunlap@infradead.org>
Reviewed-by: Nathan Lynch <nathanl@linux.ibm.com>
Signed-off-by: Michael Ellerman <mpe@ellerman.id.au>
Link: https://msgid.link/20230223070116.660-2-rdunlap@infradead.org