MIPS: add possibility to setup initial stack and global data in SRAM

This adds a new Kconfig option CONFIG_MIPS_INIT_STACK_IN_SRAM which
a SoC can select if it supports some kind of SRAM. Together with
CONFIG_SYS_INIT_SP_ADDR the initial stack and global data can be
set up in that SRAM. This can be used to provide a C environment
also for lowlevel_init().

Signed-off-by: Daniel Schwierzeck <daniel.schwierzeck@gmail.com>

arch/mips/Kconfig

@@ -302,6 +302,17 @@ config SWAP_IO_SPACE
 config SYS_MIPS_CACHE_INIT_RAM_LOAD
 	bool
 
+config MIPS_INIT_STACK_IN_SRAM
+	bool
+	default n
+	help
+	  Select this if the initial stack frame could be setup in SRAM.
+	  Normally the initial stack frame is set up in DRAM which is often
+	  only available after lowlevel_init. With this option the initial
+	  stack frame and the early C environment is set up before
+	  lowlevel_init. Thus lowlevel_init does not need to be implemented
+	  in assembler.
+
 config SYS_DCACHE_SIZE
 	int
 	default 0

arch/mips/cpu/start.S

@@ -234,6 +234,11 @@ wr_done:
 	 nop
 #endif
 
+#ifdef CONFIG_MIPS_INIT_STACK_IN_SRAM
+	/* Set up initial stack and global data */
+	setup_stack_gd
+#endif
+
 #ifndef CONFIG_SKIP_LOWLEVEL_INIT
 # ifdef CONFIG_SYS_MIPS_CACHE_INIT_RAM_LOAD
 	/* Initialize any external memory */
@@ -255,8 +260,10 @@ wr_done:
 # endif
 #endif
 
+#ifndef CONFIG_MIPS_INIT_STACK_IN_SRAM
 	/* Set up initial stack and global data */
 	setup_stack_gd
+#endif
 
 	move	a0, zero		# a0 <-- boot_flags = 0
 	PTR_LA	t9, board_init_f