powerpc: Define missing ppc_md.nvram_size for CHRP and PowerMac

Add the nvram_size() function to those PowerPC platforms that don't already
have one: CHRP and PowerMac. This means that the ppc_md.nvram_size()
function can be called by nvram_get_size().

Since we are addressing CHRP inconsistencies here, rename chrp_nvram_read
and chrp_nvram_write, which break the naming convention used across
powerpc platforms for NVRAM accessor functions.

Tested-by: Stan Johnson <userm57@yahoo.com>
Signed-off-by: Finn Thain <fthain@telegraphics.com.au>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>