ARM: dts: lpc32xx: phy3250: fix SD card regulator voltage

The fixed voltage regulator on Phytec phyCORE-LPC3250 board, which
supplies SD/MMC card's power, has a constant output voltage level
of either 3.15V or 3.3V, the actual value depends on JP4 position,
the power rail is referenced as VCC_SDIO in the board hardware manual.

Fixes: d06670e9

 ("arm: dts: phy3250: add SD fixed regulator")
Signed-off-by: Vladimir Zapolskiy <vz@mleia.com>