pmic: Rewrite the pmic command to not only work with single byte transmission

Up till now it was only possible to use 'pmic' command with a single byte
transmission.
The pmic_read|write functions has been replaced with ones, which don't need
the transmission length as a parameter.

Due to that it is possible now to read data from PMICs transmitting more
data than 1 byte at once (e.g. mc34708)

Signed-off-by: Lukasz Majewski <lukma@denx.de>
Reviewed-by: Simon Glass <sjg@chromium.org>