imx: imx6ull: adjust the ldo 1.2v bandgap voltage

Per to design team, on i.MX6UL, the LDO 1.2V bandgap voltage
is 30mV higher, so we need to adjust the REFTOP_VBGADJ(anatop
MISC0 bit[6:4]) setting to 2b'110.

Signed-off-by: Peng Fan <peng.fan@nxp.com>
Signed-off-by: Bai Ping <ping.bai@nxp.com>
Cc: Stefano Babic <sbabic@denx.de>