As slab is available early now, use kzalloc() rather than
alloc_bootmem_low() in pxa_init_gpio_chip().

This removes the following boot time warning:

<4>------------[ cut here ]------------
<4>WARNING: at mm/bootmem.c:535 alloc_arch_preferred_bootmem+0x2c/0x54()
<d>Modules linked in:
[<c0029430>] (unwind_backtrace+0x0/0xdc) from [<c0036f64>] (warn_slowpath_common+0x4c/0x80)
[<c0036f64>] (warn_slowpath_common+0x4c/0x80) from [<c000ede0>] (alloc_arch_preferred_bootmem+0x2c/0x54)
[<c000ede0>] (alloc_arch_preferred_bootmem+0x2c/0x54) from [<c000f2e4>] (___alloc_bootmem_nopanic+0x34/0xd0)
[<c000f2e4>] (___alloc_bootmem_nopanic+0x34/0xd0) from [<c000f6e4>] (___alloc_bootmem+0xc/0x34)
[<c000f6e4>] (___alloc_bootmem+0xc/0x34) from [<c000cb20>] (pxa_init_gpio+0x48/0x228)
[<c000cb20>] (pxa_init_gpio+0x48/0x228) from [<c0009794>] (init_IRQ+0x34/0x44)
[<c0009794>] (init_IRQ+0x34/0x44) from [<c00089d4>] (start_kernel+0x144/0x264)
[<c00089d4>] (start_kernel+0x144/0x264) from [<a0008034>] (0xa0008034)
<4>---[ end trace 1b75b31a2719ed1c ]---
Signed-off-by: Daniel Mack <daniel@caiaq.de>
Signed-off-by: Eric Miao <eric.y.miao@gmail.com>