qemuppc64: set the qemuppc64 nfs r/wsize mount options to 524288

On master oe, build a qemuppc64 with systemd as default init, when we
use nfs bootup, the kernel might panic due to missing symbol in dynamic
libraries as below:

  hid-generic 0003:0627:0001.0003: input: USB HID v0.01 Mouse [QEMU QEMU USB Tablet] on usb-0000:00:01.0-3/input0
  /sbin/init: /lib64/libm.so.6: version `XZ_5.0' not found (required by /usr/lib64/libkmod.so.2)
  Kernel panic - not syncing: Attempted to kill init! exitcode=0x00007f00
  CPU: 0 PID: 1 Comm: init Not tainted 5.15.78-yocto-standard #1
  Call Trace:
  [c000000007443ba0] [c0000000009538d0] dump_stack_lvl+0x74/0xa8 (unreliable)
  [c000000007443be0] [c000000000103524] panic+0x170/0x3cc
  [c000000007443c80] [c00000000010cf64] do_exit+0xb44/0xb50
  [c000000007443d50] [c00000000010d040] do_group_exit+0x60/0xd0
  [c000000007443d90] [c00000000010d0d4] sys_exit_group+0x24/0x30
  [c000000007443db0] [c00000000002cfd4] system_call_exception+0x194/0x2f0
  [c000000007443e10] [c00000000000c2cc] system_call_common+0xec/0x250
  --- interrupt: c00 at 0x7fff9ed9e840
  NIP:  00007fff9ed9e840 LR: 00007fff9ed7da20 CTR: 0000000000000000
  REGS: c000000007443e80 TRAP: 0c00   Not tainted (5.15.78-yocto-standard)
  MSR:  800000000280f033 <SF,VEC,VSX,EE,PR,FP,ME,IR,DR,RI,LE>  CR: 24022442  XER: 00000000

One or more of the libraries systemd depends on failed to load due to
unresolved symbols/functions.  This was intermittent - with a failure
rate estimated between 5% and 30%.

After checking the code, this issue happens on gcc 12, kirkstone is using
gcc 11 works well, with both using the exact same v5.15.84 kernel commit.

There is a kernel fix from upstream [1], they changed the rsize / wsize
to a multiple of PAGE_SIZE, when we applied this patch, the qemuppc64's
default r/wsize went from 4096 to 524288.But the qemuppc64 doesn't have
its own linux-yocto kernel branch, so apply this change might cause
regression with other platforms which share branch with qemuppc64.

So, we added an extra option for nfs rootfs, and set the qemuppc64 default
r/w size to 524288 to line up with the kernel fix[1].

Yocto did a similar thing in the distant past[2] - prior to boot-arg
adjustments existing - by allowing a Kconfig to set the defaults on
nfsboot, in order to work around hardware limitations.

Reference:
[1] https://git.kernel.org/pub/scm/linux/kernel/git/torvalds/linux.git/commit/?id=940261a195080cf
[2] https://git.yoctoproject.org/linux-yocto-4.1/commit/?h=standard/base&id=a96cfd98add95



Signed-off-by: Xiangyu Chen <xiangyu.chen@windriver.com>
Signed-off-by: Luca Ceresoli <luca.ceresoli@bootlin.com>