arch: arm: use cmsis functions to configure PSP and interrupts

cmsis provides intrinsic functions to configure processor special
function register like control, psp(process stack pointer). Let's use
these functions which are maintained in cmsis repository.

This patch uses cmsis function to:
	- set process stack pointer(PSP)
	- switch SP to PSP
	- disable interrupts

It also moves stack initialization from assembly to C

Signed-off-by: Vikas Manocha <vikas.manocha@st.com>