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>