ieee1275: block-size deblocker support method

ieee1275: block-size deblocker support method

IEEE Std 1275-1994 Standard for Boot (Initialization Configuration)
Firmware: Core Requirements and Practices

3.8.3 deblocker support package

Any package that uses the "deblocker" support package must define
the following method, which the deblocker uses as a low-level
interface to the device

block-size ( -- block-len ) Return "granularity" for accesses to this
device.

Signed-off-by: Eric Snowberg <eric.snowberg@oracle.com>
Reviewed-by: Daniel Kiper <daniel.kiper@oracle.com>