SecurityPkg/RngDxe: Rename RdRandGenerateEntropy to generic name

SecurityPkg/RngDxe: Rename RdRandGenerateEntropy to generic name

Bugzilla: 3668 (https://bugzilla.tianocore.org/show_bug.cgi?id=3668)

Rename RdRandGenerateEntropy() to GenerateEntropy() to provide a
common interface to generate entropy on other architectures.
GenerateEntropy() is intended to generate high quality entropy.

Also move the definition to RngDxeInternals.h

Signed-off-by: Pierre Gondois <pierre.gondois@arm.com>
Acked-by: Jiewen Yao <jiewen.yao@intel.com>