arm: cortex_m: skip clearing CONTROL if this is done at boot

If CONTROL register is done in reset.S we can skip
clearing the FPCA when enabling the floating point
support, to save a few instructions. The CONTROL
register is cleared right after boot, if the symbol
CONFIG_INIT_ARCH_HW_AT_BOOT is enabled.

Signed-off-by: Ioannis Glaropoulos <Ioannis.Glaropoulos@nordicsemi.no>