x86/microcode/AMD: Limit Entrysign signature checking to known generations

Limit Entrysign sha256 signature checking to CPUs in the range Zen1-Zen5.

X86_BUG cannot be used here because the loading on the BSP happens way
too early, before the cpufeatures machinery has been set up.

Signed-off-by: Borislav Petkov (AMD) <bp@alien8.de>
Link: https://patch.msgid.link/all/20251023124629.5385-1-bp@kernel.org