soc: arm: atmel_sam: Sys_arch_reboot using RSTC

The previous implementation of the sys_arch_reboot function
for the Atmel SAM series was using NVIC_SystemReset.
This caused a reboot time of around 20 seconds on a SAM4SA16CA,
which is now reduced by directly writing to the
reset controller control register (RSTC_CR).

Signed-off-by: Jaro Van Landschoot <jaro.vanlandschoot@basalte.be>
Co-authored-by: Gerson Fernando Budke <nandojve@gmail.com>