nios2: Don't use _end for calculating min_low_pfn

If there is a initramfs linked into the kernel which will be
freed later on there is free memory somewhere between _etext
and _end, thus using _end for min_low_pfn is not correct.
This may lead to issues in dma_capable when checking
'min(addr, end) < phys_to_dma(dev, PFN_PHYS(min_low_pfn)))'
as the address addr might be below min_low_pfn. Picked
find_limits from architecture arm for applying min_low_pfn and
max_low_pfn. Maybe using _etext for min_low_pfn would be
fine too.
Signed-off-by: Andreas Oetken <andreas.oetken@siemens.com>
Signed-off-by: Ley Foon Tan <ley.foon.tan@intel.com>

arch/nios2/kernel/setup.c

@@ -32,8 +32,6 @@ EXPORT_SYMBOL(memory_start);
 unsigned long memory_end;
 EXPORT_SYMBOL(memory_end);
 
-unsigned long memory_size;
-
 static struct pt_regs fake_regs = { 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0,
 					0, 0, 0, 0, 0, 0,
 					0};
@@ -141,16 +139,22 @@ asmlinkage void __init nios2_boot_init(unsigned r4, unsigned r5, unsigned r6,
 	parse_early_param();
 }
 
+static void __init find_limits(unsigned long *min, unsigned long *max_low,
+			       unsigned long *max_high)
+{
+	*max_low = PFN_DOWN(memblock_get_current_limit());
+	*min = PFN_UP(memblock_start_of_DRAM());
+	*max_high = PFN_DOWN(memblock_end_of_DRAM());
+}
+
 void __init setup_arch(char **cmdline_p)
 {
 	int dram_start;
 
 	console_verbose();
 
-	dram_start = memblock_start_of_DRAM();
-	memory_size = memblock_phys_mem_size();
-	memory_start = PAGE_ALIGN((unsigned long)__pa(_end));
-	memory_end = (unsigned long) CONFIG_NIOS2_MEM_BASE + memory_size;
+	memory_start = memblock_start_of_DRAM();
+	memory_end = memblock_end_of_DRAM();
 
 	init_mm.start_code = (unsigned long) _stext;
 	init_mm.end_code = (unsigned long) _etext;
@@ -161,11 +165,10 @@ void __init setup_arch(char **cmdline_p)
 	/* Keep a copy of command line */
 	*cmdline_p = boot_command_line;
 
-	min_low_pfn = PFN_UP(memory_start);
-	max_low_pfn = PFN_DOWN(memory_end);
+	find_limits(&min_low_pfn, &max_low_pfn, &max_pfn);
 	max_mapnr = max_low_pfn;
 
-	memblock_reserve(dram_start, memory_start - dram_start);
+	memblock_reserve(__pa_symbol(_stext), _end - _stext);
 #ifdef CONFIG_BLK_DEV_INITRD
 	if (initrd_start) {
 		memblock_reserve(virt_to_phys((void *)initrd_start),