watchdog: rzg2l_wdt: Fix 'BUG: Invalid wait context'

This patch fixes the issue 'BUG: Invalid wait context' during restart()
callback by using clk_prepare_enable() instead of pm_runtime_get_sync()
for turning on the clocks during restart.

This issue is noticed when testing with renesas_defconfig.

[ 42.213802] reboot: Restarting system
[ 42.217860]
[ 42.219364] =============================
[ 42.223368] [ BUG: Invalid wait context ]
[ 42.227372] 5.17.0-rc5-arm64-renesas-00002-g10393723e35e #522 Not tainted
[ 42.234153] -----------------------------
[ 42.238155] systemd-shutdow/1 is trying to lock:
[ 42.242766] ffff00000a650828 (&genpd->mlock){+.+.}-{3:3}, at: genpd_lock_mtx+0x14/0x20
[ 42.250709] other info that might help us debug this:
[ 42.255753] context-{4:4}
[ 42.258368] 2 locks held by systemd-shutdow/1:
[ 42.262806] #0: ffff80000944e1c8 (system_transition_mutex#2){+.+.}-{3:3}, at: __do_sys_reboot+0xd0/0x250
[ 42.272388] #1: ffff8000094c4e40 (rcu_read_lock){....}-{1:2}, at: atomic_notifier_call_chain+0x0/0x150
[ 42.281795] stack backtrace:
[ 42.284672] CPU: 0 PID: 1 Comm: systemd-shutdow Not tainted 5.17.0-rc5-arm64-renesas-00002-g10393723e35e #522
[ 42.294577] Hardware name: Renesas SMARC EVK based on r9a07g044c2 (DT)
[ 42.301096] Call trace:
[ 42.303538] dump_backtrace+0xcc/0xd8
[ 42.307203] show_stack+0x14/0x30
[ 42.310517] dump_stack_lvl+0x88/0xb0
[ 42.314180] dump_stack+0x14/0x2c
[ 42.317492] __lock_acquire+0x1b24/0x1b50
[ 42.321502] lock_acquire+0x120/0x3a8
[ 42.325162] __mutex_lock+0x84/0x8f8
[ 42.328737] mutex_lock_nested+0x30/0x58
[ 42.332658] genpd_lock_mtx+0x14/0x20
[ 42.336319] genpd_runtime_resume+0xc4/0x228
[ 42.340587] __rpm_callback+0x44/0x170
[ 42.344337] rpm_callback+0x64/0x70
[ 42.347824] rpm_resume+0x4e0/0x6b8
[ 42.351310] __pm_runtime_resume+0x50/0x78
[ 42.355404] rzg2l_wdt_restart+0x28/0x68
[ 42.359329] watchdog_restart_notifier+0x1c/0x30
[ 42.363943] atomic_notifier_call_chain+0x94/0x150
[ 42.368732] do_kernel_restart+0x24/0x30
[ 42.372652] machine_restart+0x44/0x70
[ 42.376399] kernel_restart+0x3c/0x60
[ 42.380058] __do_sys_reboot+0x228/0x250
[ 42.383977] __arm64_sys_reboot+0x20/0x28
[ 42.387983] invoke_syscall+0x40/0xf8

Fixes: 2cbc5cd0b55fa2 ("watchdog: Add Watchdog Timer driver for RZ/G2L")
Signed-off-by: Biju Das <biju.das.jz@bp.renesas.com>
Reviewed-by: Geert Uytterhoeven <geert+renesas@glider.be>
Reviewed-by: Guenter Roeck <linux@roeck-us.net>
Link: https://lore.kernel.org/r/20220225175320.11041-4-biju.das.jz@bp.renesas.com
Signed-off-by: Guenter Roeck <linux@roeck-us.net>
Signed-off-by: Wim Van Sebroeck <wim@linux-watchdog.org>