xtensa: call irqchip_init only when CONFIG_USE_OF is selected

During boot time kernel configured with OF=y but USE_OF=n displays the
following warnings and hangs shortly after starting userspace:

------------[ cut here ]------------
WARNING: CPU: 0 PID: 0 at kernel/irq/irqdomain.c:695 irq_create_mapping_affinity+0x29/0xc0
irq_create_mapping_affinity(, 6) called with NULL domain
CPU: 0 PID: 0 Comm: swapper Not tainted 5.15.0-rc3-00001-gd67ed2510d28 #30
Call Trace:
  __warn+0x69/0xc4
  warn_slowpath_fmt+0x6c/0x94
  irq_create_mapping_affinity+0x29/0xc0
  local_timer_setup+0x40/0x88
  time_init+0xb1/0xe8
  start_kernel+0x31d/0x3f4
  _startup+0x13b/0x13b
---[ end trace 1e6630e1c5eda35b ]---
------------[ cut here ]------------
WARNING: CPU: 0 PID: 0 at arch/xtensa/kernel/time.c:141 local_timer_setup+0x58/0x88
error: can't map timer irq
CPU: 0 PID: 0 Comm: swapper Tainted: G        W         5.15.0-rc3-00001-gd67ed2510d28 #30
Call Trace:
  __warn+0x69/0xc4
  warn_slowpath_fmt+0x6c/0x94
  local_timer_setup+0x58/0x88
  time_init+0xb1/0xe8
  start_kernel+0x31d/0x3f4
  _startup+0x13b/0x13b
---[ end trace 1e6630e1c5eda35c ]---
Failed to request irq 0 (timer)

Fix that by calling irqchip_init only when CONFIG_USE_OF is selected and
calling legacy interrupt controller init otherwise.

Fixes: da844a81 ("xtensa: add device trees support")
Signed-off-by: Max Filippov <jcmvbkbc@gmail.com>