cpu/x86/pae/pgtbl: Add memset with PAE

To clear all DRAM on x86_32, add a new method that uses PAE to access
more than 32bit of address space.
Add Documentation as well.

Required for clearing all system memory as part of security API.

Tested on wedge100s:
 Takes less than 2 seconds to clear 8GiB of DRAM.
Tested on P8H61M-Pro:
 Takes less than 1 second to clear 4GiB of DRAM.

Change-Id: I00f7ecf87b5c9227a9d58a0b61eecc38007e1a57
Signed-off-by: Patrick Rudolph <patrick.rudolph@9elements.com>
Reviewed-on: https://review.coreboot.org/c/coreboot/+/31549
Reviewed-by: Philipp Deppenwiese <zaolin.daisuki@gmail.com>
Tested-by: build bot (Jenkins) <no-reply@coreboot.org>

Documentation/arch/x86/index.md

@@ -2,6 +2,8 @@
 
 This section contains documentation about coreboot on x86 architecture.
 
+* [x86 PAE support](pae.md)
+
 ## State of x86_64 support
 At the moment there's no single board that supports x86_64 or to be exact
 `ARCH_RAMSTAGE_X86_64` and `ARCH_ROMSTAGE_X86_64`.