ARM: OMAP: correct misc 15xx and non-15xx platform code

Disable accesses to SOFT_REQ_REG2 and ULPD_SOFT_DISABLE_REQ_REG
registers for 15xx processors that don't have these registers. Enable
level 2 interrupt handler for processors that identify as OMAP 15xx
(e.g 310) and not 1510 specifically. Also fix the following compiler
warning (only visible with CONFIG_OMAP_RESET_CLOCKS):

arch/arm/mach-omap1/clock.c: In function 'omap1_clk_disable_unused':
arch/arm/mach-omap1/clock.c:634: warning: 'return' with a value, in
function returning void

Signed-off-by: Andrzej Zaborowski <balrog@zabor.org>
Signed-off-by: Tony Lindgren <tony@atomide.com>

arch/arm/mach-omap1/clock.c

@@ -496,7 +496,7 @@ static int omap1_clk_enable_generic(struct clk *clk)
 		}
 	}
 
-	return 0;
+	return;
 }
 
 static void omap1_clk_disable_generic(struct clk *clk)
@@ -654,7 +654,8 @@ int __init omap1_clk_init(void)
 	/* USB_REQ_EN will be disabled later if necessary (usb_dc_ck) */
 	reg = omap_readw(SOFT_REQ_REG) & (1 << 4);
 	omap_writew(reg, SOFT_REQ_REG);
-	omap_writew(0, SOFT_REQ_REG2);
+	if (!cpu_is_omap15xx())
+		omap_writew(0, SOFT_REQ_REG2);
 
 	clk_init(&omap1_clk_functions);