mrf24j40: add regmap support

This patch introduce regmap support for short and long address space of
mrf24j40. It's only possible to use regmap_read/write/update_bits for
long address range. This is because I added lowlevel bus operation
because the write operation need to set the 12th bit to mark a register
write, but regmap only supports to set bits for register write access in
the first byte. We use other regmap register functions than
read/write/update_bits, so this should be fine.

Reviewed-by: Stefan Schmidt <stefan@osg.samsung.com>
Signed-off-by: Alexander Aring <alex.aring@gmail.com>
Signed-off-by: Marcel Holtmann <marcel@holtmann.org>