Remove the reliance on a staticly defined NVRAM size, allowing
platforms to support NVRAMs with sizes differing from the standard.

A fall back value is provided for platforms not supporting this extension.
Signed-off-by: NMartyn Welch <martyn.welch@gefanuc.com>
Signed-off-by: NBenjamin Herrenschmidt <benh@kernel.crashing.org>