PXA: Rework start.S to be closer to other ARMs

The start.S on PXA was very obscure. This reworks it back to be close to arm1136
start.S and others.

Signed-off-by: Marek Vasut <marek.vasut@gmail.com>
Cc: Albert ARIBAUD <albert.u.boot@aribaud.net>

V2: Don't compile in relocation support if building SPL
2 files changed