booti: Set images.os.arch

Commit ec6617c3 ("armv8: Support loading 32-bit OS in AArch32
execution state") broke SMP boot by assuming that an image is 32-bit if
the arch field in the spin table != IH_ARCH_DEFAULT (i.e.
IH_ARCH_ARM64), even if the arch field also does not match IH_ARCH_ARM,
even though nothing actually set the arch field in the spin table.

Commit e2c18e40 ("armv8: fsl-layerscape: SMP support for loading
32-bit OS") fixed this for bootm by setting the arch field of the spin
table based on images.os.arch, but booti remaineed broken because it did
not set images.os.arch.

Fixes: ec6617c3 ("armv8: Support loading 32-bit OS in AArch32 execution state")
Fixes: e2c18e40 ("armv8: fsl-layerscape: SMP support for loading 32-bit OS")
Cc: Alison Wang <alison.wang@nxp.com>
Cc: Chenhui Zhao <chenhui.zhao@nxp.com>
Cc: York Sun <york.sun@nxp.com>
Cc: Stuart Yoder <stuart.yoder@nxp.com>
Signed-off-by: Scott Wood <oss@buserror.net>
Reviewed-by: Tom Rini <trini@konsulko.com>