Blackfin: fix accidental reset in some boot modes

We read the SWRST (Software Reset) register to get at the last reset
state, and then we may configure the DOUBLE_FAULT bit to control behavior
when a double fault occurs.  But if the lower bits of the register is
already set (like UART boot mode on a BF54x), we inadvertently make the
system reset by writing to the SYSTEM_RESET field at the same time.  So
make sure the lower 4 bits are always cleared.
Signed-off-by: Sonic Zhang <sonic.zhang@analog.com>
Signed-off-by: Mike Frysinger <vapier@gentoo.org>

arch/blackfin/kernel/setup.c

@@ -837,7 +837,8 @@ void __init setup_arch(char **cmdline_p)
     defined(CONFIG_BF538) || defined(CONFIG_BF539)
 	_bfin_swrst = bfin_read_SWRST();
 #else
-	_bfin_swrst = bfin_read_SYSCR();
+	/* Clear boot mode field */
+	_bfin_swrst = bfin_read_SYSCR() & ~0xf;
 #endif
 
 #ifdef CONFIG_DEBUG_DOUBLEFAULT_PRINT