arm64: KGDB: Add step debugging support

Add KGDB software step debugging support for EL1 debug
in AArch64 mode.

KGDB registers step debug handler with debug monitor.
On receiving 'step' command from GDB tool, target enables
software step debugging and step address is updated in ELR.

Software Step debugging is disabled when 'continue' command
is received

Signed-off-by: Vijaya Kumar K <Vijaya.Kumar@caviumnetworks.com>
Reviewed-by: Will Deacon <will.deacon@arm.com>
Signed-off-by: Catalin Marinas <catalin.marinas@arm.com>