arc: implement slave cores kick-start for Linux kernel

With new SMP-enabled CPUs with ARC HS38 cores and corresponding support
in Linux kernel it's required to add basic SMP support in U-Boot.

Currently we assume the one and only core starts execution after
power-on. So most of things in U-Boot is handled in UP mode.

But when U-Boot is used for loading and starting Linux kernel right
before jumping to kernel's entry point U-Boot:
 [1] Sets all slave cores to jump to the same address [kernel's entry
point]
 [2] Really starts all slav cores

In ARC's implemetation of SMP in Linux kernel all cores are supposed to
run the same start-up code. But only core with ID 0 (master core)
processes further while others are looping waiting for master core to
complete some initialization.

That means it's safe to un-pause slave cores and let them execute kernel
- they will wait for master anyway.

Signed-off-by: Alexey Brodkin <abrodkin@synopsys.com>
Cc: Vineet Gupta <vgupta@synopsys.com>

arch/arc/lib/bootm.c

@@ -53,6 +53,9 @@ static void boot_prep_linux(bootm_headers_t *images)
 		hang();
 }
 
+__weak void smp_set_core_boot_addr(unsigned long addr, int corenr) {}
+__weak void smp_kick_all_cpus(void) {}
+
 /* Subcommand: GO */
 static void boot_jump_linux(bootm_headers_t *images, int flag)
 {
@@ -80,6 +83,9 @@ static void boot_jump_linux(bootm_headers_t *images, int flag)
 		r2 = (unsigned int)getenv("bootargs");
 	}
 
+	smp_set_core_boot_addr((unsigned long)kernel_entry, -1);
+	smp_kick_all_cpus();
+
 	if (!fake)
 		kernel_entry(r0, 0, r2);
 }