powerpc/prom_init: Pass linux_banner to firmware via option vector 7

Pass the value of linux_banner to firmware via option vector 7.

Option vector 7 is described in "LoPAR" Linux on Power Architecture
Reference v2.9, in table B.7 on page 824:

  An ASCII character formatted null terminated string that describes
  the client operating system. The string shall be human readable and
  may be displayed on the console.

The string can be up to 256 bytes total, including the nul terminator.

linux_banner contains lots of information, and should make it possible
to identify the exact kernel version that is running:

  const char linux_banner[] =
  "Linux version " UTS_RELEASE " (" LINUX_COMPILE_BY "@"
  LINUX_COMPILE_HOST ") (" LINUX_COMPILER ") " UTS_VERSION "\n";

For example:
  Linux version 4.15.0-144-generic (buildd@bos02-ppc64el-018) (gcc
  version 7.5.0 (Ubuntu 7.5.0-3ubuntu1~18.04)) #148-Ubuntu SMP Sat May 8
  02:32:13 UTC 2021 (Ubuntu 4.15.0-144.148-generic 4.15.18)

It's also printed at boot to the console/dmesg, which should make it
possible to correlate what firmware receives with the console/dmesg on
the machine.

Signed-off-by: Michael Ellerman <mpe@ellerman.id.au>
Link: https://lore.kernel.org/r/20210621064938.2021419-2-mpe@ellerman.id.au