omap: common: add kernel API to query RAM size

Add kernel api omap_total_ram_size() to query the
exact size of physical ram available in system.
Its derived from mem arguments passed to kernel via
bootargs and is expected to be used by board files and drivers.
Note: omap_init_ram_size() has to be invoked at the earliest
during board init to update memory size.

Change-Id: I8511fd66ba51343bc64a58db04994af930cbe709
Signed-off-by: Devaraj Rangasamy <dev@ti.com>

Conflicts:
	arch/arm/mach-omap2/board-4430sdp.c
	arch/arm/mach-omap2/board-44xx-tablet.c
	arch/arm/mach-omap2/board-omap4panda.c