powerpc/crypto: Add generated P8 asm to .gitignore

Looks like drivers/crypto/vmx/.gitignore should have been merged into
arch/powerpc/crypto/.gitignore as part of commit
10930333

 ("crypto: vmx - Move to arch/powerpc/crypto") so that all
generated asm files are ignored.

Signed-off-by: Nathan Lynch <nathanl@linux.ibm.com>
Fixes: 10930333

 ("crypto: vmx - Move to arch/powerpc/crypto")
Signed-off-by: Michael Ellerman <mpe@ellerman.id.au>
Link: https://msgid.link/20240603-powerpc-crypto-ignore-p8-asm-v1-1-05843fec2bb7@linux.ibm.com