sandbox: dts: Add missing sandbox, emul phandle to sandbox-pmic

sandbox: dts: Add missing sandbox, emul phandle to sandbox-pmic

This phandle was missing in the sandbox DT, add it, otherwise sandbox-i2c
driver cannot look up the emulator via i2c_emul_find(). This fixes the
following i2c_emul_find() error:

"
$ ./u-boot -Dc ""
...
       i2c_emul_find() No emulators for device 'sandbox_pmic'
  sandbox_pmic_write() write error to device: 0000000018c568d0 register: 0x0!
       out_set_value() PMIC write failed: -5
       i2c_emul_find() No emulators for device 'sandbox_pmic'
  sandbox_pmic_write() write error to device: 0000000018c568d0 register: 0x0!
       out_set_value() PMIC write failed: -5
...
"

Signed-off-by: Marek Vasut <marex@denx.de>
Reviewed-by: Heiko Schocher <hs@denx.de>
Reviewed-by: Simon Glass <sjg@chromium.org>