x86/sev: Create snp_prepare()

In preparation for delayed SNP initialization, create a function snp_prepare()
that does the necessary architecture setup.  Export this function for the ccp
module to allow it to do the setup as necessary.

Introduce a cpu_read_lock/unlock() wrapper around the MFDM and SNP enable.
While CPU hotplug is not supported, this makes sure that the bit setting
happens on the same set of CPUs in both cases.

This improvement was suggested by Sashiko:

  https://sashiko.dev/#/patchset/20260324161301.1353976-1-tycho%40kernel.org

Also move {mfd,snp}_enable() out of the __init section, since these will be
called later.

Signed-off-by: Tycho Andersen (AMD) <tycho@kernel.org>
Signed-off-by: Borislav Petkov (AMD) <bp@alien8.de>
Reviewed-by: Tom Lendacky <thomas.lendacky@amd.com>
Link: https://patch.msgid.link/20260326161110.1764303-3-tycho@kernel.org

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