ARM: rockchip: select ARMv7 compiler flags for platsmp.o

When compiling for multiplatform for both ARMv6 and ARMv7, the default
compiler flags are for ARMv6, and the following cpu-hotplug change will
fail with:
 /tmp/ccSFxfmI.s:68: Error: selected processor does not support ARM mode `isb '
 /tmp/ccSFxfmI.s:74: Error: selected processor does not support ARM mode `isb '
 /tmp/ccSFxfmI.s:75: Error: selected processor does not support ARM mode `dsb '

Fix this in a similar manner as in commit 9f0affcf "ARM: mvebu: Fix pmsu
compilation when ARMv6 is selected", by specifying ARMv7 flags for platsmp.o.

Signed-off-by: Heiko Stuebner <heiko@sntech.de>