powerpc: Fix mpic alloc warning

Since we can use kmalloc earlier we are getting the following since the
mpic_alloc() code calls alloc_bootmem().  Move to using kzalloc() to
remove the warning.

------------[ cut here ]------------
Badness at c0583248 [verbose debug info unavailable]
NIP: c0583248 LR: c0583210 CTR: 00000004
REGS: c0741de0 TRAP: 0700   Not tainted  (2.6.30-06736-g12a31df)
MSR: 00021000 <ME,CE>  CR: 22024024  XER: 00000000
TASK = c070d3b8[0] 'swapper' THREAD: c0740000 CPU: 0
<6>GPR00: 00000001 c0741e90 c070d3b8 00000001 00000210 00000020 3fffffff 00000000
<6>GPR08: 00000000 c0c85700 c04f8c40 0000002d 22044022 1004a388 7ffd9400 00000000
<6>GPR16: 00000000 7ffcd100 7ffcd100 7ffcd100 c04f8c40 00000000 c059f62c c075a0c0
<6>GPR24: c059f648 00000000 0000000f 00000210 00000020 00000000 3fffffff 00000210
NIP [c0583248] alloc_arch_preferred_bootmem+0x50/0x80
LR [c0583210] alloc_arch_preferred_bootmem+0x18/0x80
Call Trace:
[c0741e90] [c07343b0] devtree_lock+0x0/0x24 (unreliable)
[c0741ea0] [c0583b14] ___alloc_bootmem_nopanic+0x54/0x108
[c0741ee0] [c0583e18] ___alloc_bootmem+0x18/0x50
[c0741ef0] [c057b9cc] mpic_alloc+0x48/0x710
[c0741f40] [c057ecf4] mpc85xx_ds_pic_init+0x190/0x1b8
[c0741f90] [c057633c] init_IRQ+0x24/0x34
[c0741fa0] [c05738b8] start_kernel+0x260/0x3dc
[c0741ff0] [c00003c8] skpinv+0x2e0/0x31c
Instruction dump:
409e001c 7c030378 80010014 83e1000c 38210010 7c0803a6 4e800020 3d20c0c8
39295700 80090004 7c000034 5400d97e <0f000000> 2f800000 409e001c 38800000

BenH: Changed to use GFP_KERNEL, the allocator will do the right thing

Signed-off-by: Kumar Gala <galak@kernel.crashing.org>
Signed-off-by: Benjamin Herrenschmidt <benh@kernel.crashing.org>