ppc: Remove duplicate exports of __down, __up etc.

__down, __down_interruptible and __up are defined and exported in
arch/powerpc/kernel/semaphore.c, and used from there for ARCH=ppc,
so there is no need to export them in arch/ppc/kernel/ppc_ksyms.c.

Signed-off-by: Paul Mackerras <paulus@samba.org>