yp-r0: improve the charging code

Some people reported strange charging times and
strange battery life.
Charging by OF: 25 hours; RB: 18 hours
It has been found that there are at least two issues here:

1) the way of getting battery charging status wasn't
really accurate. This attempts to fix that issue.
This patch also simplifies some code (opening a
device is no more needed, for example).

To technically explain, battery charging implies first a constant
current mode (where the voltage increases) and then a constant
voltage mode (where, obviously, the voltage reads more or less
the same). The old way the End Of Charge was detected was based solely
on the voltage, while now it is based on chip's EOC interrupt,
which should be more accurate.

2) OF explicitly sets a constant current 350 mA, while we usually had
55 mA (by as3543 default).
This wasn't discovered before since there is a caching problem
("an accurate guess") in the Samsung power mgmt Linux module,
thus its debugging dumper wasn't really working, reporting a 55 mA current.
Strangely this option should have been set by the bootloader
but apparently it does not.

Some testing is still needed, but I confirm that with this patch
I could run a benchmark for 25 hours (vs. 18 hours), by charging
within Rockbox of course.

Change-Id: I3bd921e86b9018d1cc3c720d15cc46896e8490b3

firmware/export/config/samsungypr0.h

@@ -122,11 +122,6 @@
 
 #endif /* SIMULATOR */
 
-/* FIXME
- * Lot of people reports bad battery life and funny charging times.
- * Check what's going on...
- */
-
 #define BATTERY_CAPACITY_DEFAULT 600 /* default battery capacity */
 #define BATTERY_CAPACITY_MIN 600  /* min. capacity selectable */
 #define BATTERY_CAPACITY_MAX 600 /* max. capacity selectable */
@@ -138,6 +133,9 @@
 /* Linux controlls charging, we can monitor */
 #define CONFIG_CHARGING CHARGING_MONITOR
 
+/* We want to be able to reset the averaging filter */
+#define HAVE_RESET_BATTERY_FILTER
+
 /* same dimensions as gigabeats */
 #define CONFIG_LCD LCD_YPR0