boards: arm: atsame54_xpro: Use emulation reset for OpenOCD

The OpenOCD JTAG reset causes the SAM E54 SoC to prematurely start code
execution before the SoC is ready, and this causes hard faults when the
MPU is enabled -- causing OpenOCD to hang.

This commit disables the JTAG SRST reset mechanism and falls back to
using the Cortex-M SYSRESETREQ emulation reset mechanism, which does
not have experience this issue.

Signed-off-by: Stephanos Ioannidis <root@stephanos.io>