powerpc: define the conditions where the ePAPR idle hcall can be supported

For 32-bit, CONFIG_EPAPR_PARAVIRT pulls in both epapr_paravirt.c
and epapr_hcalls.c which contains the 32-bit paravirt idle loop.

For 64-bit, the paravirt idle loop is in idle_book3e.S and that
source file is included only if CONFIG_PPC_BOOK3E_64 defined.

This patch makes that dependency for 64-bit explicit.

Fixes these build errors:

arch/powerpc/kernel/built-in.o: In function `restore_pblist_ptr':
ftrace.c:(.toc+0xdc0): undefined reference to `epapr_ev_idle_start'
ftrace.c:(.toc+0xdd0): undefined reference to `epapr_ev_idle'

Signed-off-by: Stuart Yoder <stuart.yoder@freescale.com>
Signed-off-by: Stephen Rothwell <sfr@canb.auug.org.au>