kernel: arm: Increase idle stack size to fix corruption by FP_SHARING

When enabling CONFIG_FP_SHARING on ARM, 64 extra bytes are necessary
on the stack of each task in order to save FPU registers S16 to S31.

In the case of the idle stack, the default value of 256 bytes is too
small. As described in ZEP-1470, when the idle task is scheduled out,
floating point registers are saved, which corrupts the stack frame
(especially the saved PC value). When scheduling the idle task, the
restored PC will jump to nowhere, leading to a Usage Fault.

Increase the size of the idle stack by 64 bytes to fix this issue.

JIRA: ZEP-1470

Change-Id: Ib800cd51e5189dda8bf59332db661c21399db3e3
Signed-off-by: Florian Vaussard <florian.vaussard@heig-vd.ch>
Signed-off-by: Kumar Gala <kumar.gala@linaro.org>