riscv: Set text_offset correctly for M-Mode

M-Mode Linux is loaded at the start of RAM, not 2MB later. Perhaps this
should be calculated based on PAGE_OFFSET somehow? Even better would be to
deprecate text_offset and instead introduce something absolute.

Signed-off-by: Sean Anderson <seanga2@gmail.com>
Signed-off-by: Palmer Dabbelt <palmerdabbelt@google.com>

arch/riscv/kernel/head.S

@@ -35,12 +35,17 @@ ENTRY(_start)
 	.word 0
 #endif
 	.balign 8
+#ifdef CONFIG_RISCV_M_MODE
+	/* Image load offset (0MB) from start of RAM for M-mode */
+	.dword 0
+#else
 #if __riscv_xlen == 64
 	/* Image load offset(2MB) from start of RAM */
 	.dword 0x200000
 #else
 	/* Image load offset(4MB) from start of RAM */
 	.dword 0x400000
+#endif
 #endif
 	/* Effective size of kernel image */
 	.dword _end - _start