microblaze: Fix _start symbol to physical address

_start is setup to physical kernel start address.
This caused that when you load vmlinux (with MMU kernel)
via XMD program counter (pc) is setup correctly
and then you can write con and start kernel.
Signed-off-by: Michal Simek <monstr@monstr.eu>

arch/microblaze/kernel/vmlinux.lds.S

@@ -20,6 +20,7 @@ jiffies = jiffies_64 + 4;
 
 SECTIONS {
 	. = CONFIG_KERNEL_START;
+	_start = CONFIG_KERNEL_BASE_ADDR;
 	.text : AT(ADDR(.text) - LOAD_OFFSET) {
 		_text = . ;
 		_stext = . ;