client: Load initramdisk location

For ages both vmlinux and zImage accepted the initramdisk location
in r3/r4 [1] [2]. If r3==r4==0, vmlinux looks at the device tree for
/chosen/linux,initrd-{start|end} but zImage does not so the QEMU user
can only pass vmlinux via -kernel if -initrd is passed as well.

This initializes r3/r4 to point to the initramdisk location when present.

[1] https://git.kernel.org/pub/scm/linux/kernel/git/torvalds/linux.git/tree/arch/powerpc/kernel/prom_init.c?h=v5.3#n3230
[2] https://git.kernel.org/pub/scm/linux/kernel/git/torvalds/linux.git/tree/arch/powerpc/boot/of.c?h=v5.3#n89

Signed-off-by: Alexey Kardashevskiy <aik@ozlabs.ru>
---
Changes:
v2:
* improved readability

slof/fs/client.fs

@@ -45,6 +45,14 @@ VARIABLE  client-callback \ Address of client's callback function
   >r  ciregs >r7 !  ciregs >r6 !  client-entry-point @ ciregs >r5 !
   \ Initialise client-stack-pointer
   cistack ciregs >r1 !
+
+  s" linux,initrd-end" get-chosen IF decode-int nip nip ELSE 0 THEN
+  s" linux,initrd-start" get-chosen IF decode-int nip nip ELSE 0 THEN
+		    ( end start )
+  tuck -	    ( start len )
+  ciregs >r4 !
+  ciregs >r3 !
+
   \ jump-client maps to call_client in slof/entry.S
   \ When jump-client returns, R3 holds the address of a NUL-terminated string
   \ that holds the client interface word the client wants to call, R4 holds