powerpc/ps3: Add firmware version to sysfs

Add a new sysfs entry /sys/firmware/ps3/fw-version that exports
the PS3's firmware version.

The firmware version is available through an LV1 hypercall, and we've
been printing it to the boot log, but haven't provided an easy way for
user utilities to get it.

Signed-off-by: Geoff Levand <geoff@infradead.org>
Signed-off-by: Michael Ellerman <mpe@ellerman.id.au>
Link: https://lore.kernel.org/r/41509b2da647cd34b1331cc4756c8774b1e284eb.1622822173.git.geoff@infradead.org