ARM: omap3_beagle: Power on MMC when setting up PMIC

The PMIC enables power to the MMC card by default, but depending
on the state it was left when restarted, it's possible the MMC
may be powered down.

This patch patch explicitly tells the twl4030 to power the MMC.

Based on commits [1][2].

[1] 64fd2d26
[2] 27b65344

Signed-off-by: Romain Naour <romain.naour@gmail.com>