cpu/x86: expose and add paging helper functions

cpu/x86: expose and add paging helper functions

Add the following functions for use outside of the paging module:

void paging_enable_pae_cr3(uintptr_t cr3);
void paging_enable_pae(void);
void paging_disable_pae(void);

The functions just enable and/or disable paging along with PAE.
Disassembly shows equivalent output for both versions.

BUG=b:72728953

Change-Id: I9665e7ec4795a5f52889791f73cf98a8f4def827
Signed-off-by: Aaron Durbin <adurbin@chromium.org>
Reviewed-on: https://review.coreboot.org/25714
Tested-by: build bot (Jenkins) <no-reply@coreboot.org>
Reviewed-by: Furquan Shaikh <furquan@google.com>
Reviewed-by: Justin TerAvest <teravest@chromium.org>