ARM: OMAP2+: AM43x: resolve SMP related build error

If AM43x and SMP is selected, OMAP4 & OMAP5 deselected, build error as
follows,

arch/arm/mach-omap2/built-in.o: In function `scu_gp_set':
arch/arm/mach-omap2/sleep44xx.S:131: undefined reference to `omap4_get_scu_base'
arch/arm/mach-omap2/sleep44xx.S:132: undefined reference to `scu_power_mode'
arch/arm/mach-omap2/built-in.o: In function `scu_gp_clear':
arch/arm/mach-omap2/sleep44xx.S:227: undefined reference to `omap4_get_scu_base'
arch/arm/mach-omap2/sleep44xx.S:229: undefined reference to `scu_power_mode'

Resolve it by building sleep44xx.S only for OMAP4 & OMAP5.

Signed-off-by: Afzal Mohammed <afzal@ti.com>
Signed-off-by: Arnd Bergmann <arnd@arndb.de>