x86/efi: Allow mapping BGRT on x86-32

x86/efi: Allow mapping BGRT on x86-32

CONFIG_X86_32 doesn't map the boot services regions into the EFI memory
map (see commit 700870119f49 ("x86, efi: Don't map Boot Services on
i386")), and so efi_lookup_mapped_addr() will fail to return a valid
address. Executing the ioremap() path in efi_bgrt_init() causes the
following warning on x86-32 because we're trying to ioremap() RAM,

 WARNING: CPU: 0 PID: 0 at arch/x86/mm/ioremap.c:102 __ioremap_caller+0x2ad/0x2c0()
 Modules linked in:
 CPU: 0 PID: 0 Comm: swapper/0 Not tainted 3.13.0-0.rc5.git0.1.2.fc21.i686 #1
 Hardware name: DellInc. Venue 8 Pro 5830/09RP78, BIOS A02 10/17/2013
  00000000 00000000 c0c0df08 c09a5196 00000000 c0c0df38 c0448c1e c0b41310
  00000000 00000000 c0b37bc1 00000066 c043bbfd c043bbfd 00e7dfe0 00073eff
  00073eff c0c0df48 c0448ce2 00000009 00000000 c0c0df9c c043bbfd 00078d88
 Call Trace:
  [<c09a5196>] dump_stack+0x41/0x52
  [<c0448c1e>] warn_slowpath_common+0x7e/0xa0
  [<c043bbfd>] ? __ioremap_caller+0x2ad/0x2c0
  [<c043bbfd>] ? __ioremap_caller+0x2ad/0x2c0
  [<c0448ce2>] warn_slowpath_null+0x22/0x30
  [<c043bbfd>] __ioremap_caller+0x2ad/0x2c0
  [<c0718f92>] ? acpi_tb_verify_table+0x1c/0x43
  [<c0719c78>] ? acpi_get_table_with_size+0x63/0xb5
  [<c087cd5e>] ? efi_lookup_mapped_addr+0xe/0xf0
  [<c043bc2b>] ioremap_nocache+0x1b/0x20
  [<c0cb01c8>] ? efi_bgrt_init+0x83/0x10c
  [<c0cb01c8>] efi_bgrt_init+0x83/0x10c
  [<c0cafd82>] efi_late_init+0x8/0xa
  [<c0c9bab2>] start_kernel+0x3ae/0x3c3
  [<c0c9b53b>] ? repair_env_string+0x51/0x51
  [<c0c9b378>] i386_start_kernel+0x12e/0x131

Switch to using early_memremap(), which won't trigger this warning, and
has the added benefit of more accurately conveying what we're trying to
do - map a chunk of memory.

This patch addresses the following bug report,

  https://bugzilla.kernel.org/show_bug.cgi?id=67911

Reported-by: Adam Williamson <awilliam@redhat.com>
Cc: Josh Triplett <josh@joshtriplett.org>
Cc: Matthew Garrett <mjg59@srcf.ucam.org>
Cc: Rafael J. Wysocki <rjw@rjwysocki.net>
Signed-off-by: Matt Fleming <matt.fleming@intel.com>