nrf51: Rename to nrf5

nrf51: Rename to nrf5

Renaming of all nrf51 NOR flash code to nrf5, as to prepare the
code for being able to flash nrf51 and nrf52 chips.

The nrf51 command is retained for backwards compatability.

"nRF5" is also the name Nordic Semiconductor uses to describe
both the nrf51 and nrf52 chips.

Change-Id: I5f4e3f1ec780184b28ad44f735a746e68908c502
Signed-off-by: Slowcoder <slowcoder@gmail.com>
Reviewed-on: http://openocd.zylin.com/4209
Tested-by: jenkins
Reviewed-by: Tomas Vanek <vanekt@fbl.cz>
Reviewed-by: Spencer Oliver <spen@spen-soft.co.uk>