lib: sbi_ipi: Move initial IPI clear to sbi_ipi_init()

sbi_ipi_init() expects the platform warm init function to clear IPIs
on the local hart, but there is already a generic function to do this.
After this change, none of the existing drivers need a warm init
callback.

Signed-off-by: Samuel Holland <samuel.holland@sifive.com>
Reviewed-by: Anup Patel <anup@brainfault.org>

lib/sbi/sbi_hsm.c

@@ -178,7 +178,7 @@ static void sbi_hsm_hart_wait(struct sbi_scratch *scratch)
 
 	/*
 	 * No need to clear IPI here because the sbi_ipi_init() will
-	 * clear it for current HART via sbi_platform_ipi_init().
+	 * clear it for current HART.
 	 */
 }