arm: omap-common: Guard some parts of the code with CONFIG_OMAP44XX/OMAP54XX

On OMAP4 platforms that also need to calculate their DDR settings we are
now getting very close to the linker limit size.  Since OMAP44XX is only
seen with LPDDR2, remove some run time tests for LPDDR2 or DDR3 as we
will know that we don't have it for OMAP44XX.

Cc: Nishanth Menon <nm@ti.com>
Signed-off-by: Tom Rini <trini@konsulko.com>