gdbserver: Implement p and P packets

gdbserver: Implement p and P packets

GDB will need the p packet to individually request "load early" registers
before using the g packet.

Not sure if P is necessary, but if p is supported, why not implement P?

Alternatively, to be more efficient there could be a packet where GDB can
specify a list of registers it wants do load or set.  Or there could be a
register to request/set the expedited registers.

Reviewed-By: Luis Machado <luis.machado@arm.com>