ARM: scu: provide inline dummy functions when SCU is not present

On platforms such as Cortex-A15 based OMAP5, SCU is not used, however
since much code is shared between Cortex-A9 based OMAP4 (which uses
SCU) and OMAP5, It does help to have inline functions returning error
values when SCU is not present on the platform.

arch/arm/mach-omap2/omap-smp.c which is common between OMAP4 and 5
handles the SCU usage only for OMAP4.

This fixes the following build failure with OMAP5 only build:
arch/arm/mach-omap2/built-in.o: In function `omap4_smp_init_cpus':
arch/arm/mach-omap2/omap-smp.c:185: undefined reference to `scu_get_core_count'
arch/arm/mach-omap2/built-in.o: In function `omap4_smp_prepare_cpus':
arch/arm/mach-omap2/omap-smp.c:211: undefined reference to `scu_enable'

Reported-by: Pekon Gupta <pekon@ti.com>
Reported-by: Vincent Stehlé <v-stehle@ti.com>
Acked-by: Santosh Shilimkar <santosh.shilimkar@ti.com>
Signed-off-by: Nishanth Menon <nm@ti.com>
Signed-off-by: Tony Lindgren <tony@atomide.com>