crypto/armv8: replace meson option with pkg-config support

crypto/armv8: replace meson option with pkg-config support

With pkg-config support available within AArch64crypto library,
meson option 'armv8_crypto_dir' can be removed.
PKG_CONFIG_PATH environment variable should be set appropriately
to use the crypto library.

Suggested-by: Thomas Monjalon <thomas@monjalon.net>
Signed-off-by: Dharmik Thakkar <dharmik.thakkar@arm.com>
Reviewed-by: Ruifeng Wang <ruifeng.wang@arm.com>
Acked-by: Bruce Richardson <bruce.richardson@intel.com>