firmware: imx: SCU irq should ONLY be enabled after SCU IPC is ready

The imx_scu_irq_group_enable() is normally called during module driver
probe phase to enable SCU group irq, if SCU IPC is NOT ready, below
dump will show out:

[    0.933001] Hardware name: Freescale i.MX8QXP MEK (DT)
[    0.938129] pstate: 60000005 (nZCv daif -PAN -UAO)
[    0.942907] pc : imx_scu_call_rpc+0x114/0x158
[    0.947251] lr : imx_scu_irq_group_enable+0x74/0xc4
[    0.952113] sp : ffff00001005bae0
[    0.955415] x29: ffff00001005bae0 x28: ffff0000111bb0a0
[    0.960712] x27: ffff00001140b000 x26: ffff00001111068c
[    0.966011] x25: ffff0000111bb100 x24: 0000000000000000
[    0.971311] x23: ffff0000113d9cd8 x22: 0000000000000001
[    0.976610] x21: 0000000000000001 x20: ffff80083b51a410
[    0.981909] x19: ffff000011259000 x18: 0000000000000480
[    0.987209] x17: 000000000023ffb8 x16: 0000000000000010
[    0.992508] x15: 000000000000023f x14: ffffffffffffffff
[    0.997807] x13: 0000000000000018 x12: 0000000000000030
[    1.003107] x11: 0000000000000003 x10: 0101010101010101
[    1.008406] x9 : ffffffffffffffff x8 : 7f7f7f7f7f7f7f7f
[    1.013706] x7 : fefefeff646c606d x6 : 0000000000000000
[    1.019005] x5 : ffff0000112596c8 x4 : 0000000000000008
[    1.024304] x3 : 0000000000000003 x2 : 0000000000000001
[    1.029604] x1 : ffff00001005bb58 x0 : 0000000000000000
[    1.034905] Call trace:
[    1.037341]  imx_scu_call_rpc+0x114/0x158
[    1.041334]  imx_scu_irq_group_enable+0x74/0xc4
[    1.045856]  imx_sc_wdt_probe+0x24/0x150
[    1.049766]  platform_drv_probe+0x4c/0xb0
[    1.053762]  really_probe+0x1f8/0x2c8
[    1.057407]  driver_probe_device+0x58/0xfc
[    1.061490]  device_driver_attach+0x68/0x70
[    1.065660]  __driver_attach+0x94/0xdc
[    1.069397]  bus_for_each_dev+0x64/0xc0
[    1.073220]  driver_attach+0x20/0x28
[    1.076782]  bus_add_driver+0x148/0x1fc
[    1.080601]  driver_register+0x68/0x120
[    1.084424]  __platform_driver_register+0x4c/0x54
[    1.089120]  imx_sc_wdt_driver_init+0x18/0x20
[    1.093463]  do_one_initcall+0x58/0x1b8
[    1.097287]  kernel_init_freeable+0x1cc/0x288
[    1.101630]  kernel_init+0x10/0x100
[    1.105101]  ret_from_fork+0x10/0x18
[    1.108669] ---[ end trace 9e03302114457de9 ]---
[    1.113296] enable irq failed, group 1, mask 1, ret -22

To avoid such scenario, return -EPROBE_DEFER in imx_scu_irq_group_enable()
API if SCU IPC is NOT ready, then module driver which calls this API
in probe phase will defer probe after SCU IPC ready.

Fixes: 851826c7 ("firmware: imx: enable imx scu general irq function")
Signed-off-by: Anson Huang <Anson.Huang@nxp.com>
Signed-off-by: Shawn Guo <shawnguo@kernel.org>