arm: Enable -ffunction-sections / -fdata-sections / --gc-sections

While other architectures have enabled these gcc / ld options for some
time on U-Boot itself, ARM has only been doing this on SPL.  Enable this
on full U-Boot as well now.

Cc: Albert ARIBAUD <albert.u.boot@aribaud.net>
Signed-off-by: Tom Rini <trini@ti.com>