omap4/5/am33xx: Make lowlevel_init available to all armv7 platforms

Make the lowlevel_init function that these platforms have which just
sets up the stack and calls a C function available to all armv7
platforms.  As part of this we change some of the macros that are used
to be more clear.  Previously (except for am335x evm) we had been
setting CONFIG_SYS_INIT_SP_ADDR to a series of new defines that are
equivalent to simply referencing NON_SECURE_SRAM_END.  On am335x evm we
should have been doing this initially and do now.

Cc: Sricharan R <r.sricharan@ti.com>
Tested-by: Allen Martin <amartin@nvidia.com>
Signed-off-by: Tom Rini <trini@ti.com>