Rewrite the Bathroom Epiphanies Frosty Flake matrix and LED handling (#8243)

* Keyboard: revamp frosty-flake leds

This commit transitions bpiphany/frosty_flake to led_update_{kb,user}
and rewrites the AVR bit twiddling logic to use the standard QMK GPIO
API.

* Keyboard: rewrite frosty_flake's matrix reader to be a lite custom matrix

This commit replaces frosty_flake's custom matrix and debounce logic
with a "lite" custom matrix. In addition to being somewhat clearer, this
allows a consumer of the flake board to choose their own debouncing
algorithm. The one closest to the implementation originally in use is
sym_g, but this opens us up to supporting eager_pk and eager_pr.

The original matrix code was 18 columns for 8 rows, but using a single
row read and unpacking the bits into individual columns. To simplify,
I've changed the key layout to be 8C 18R instead of 18C 8R: this lets us
use a single read directly into the matrix _and_ drop down to a uint8_t
instead of a uint32_t for matrix_row_t.

Since we're no longer implementing our own debouncing and row unpacking,
we save ~400 bytes on the final firmware image.

Fully tested against a CM Storm QFR hosting the flake -- this commit
message was written using the new matrix code.

Firmware Sizes (assuming stock configuration as of 42d6270f2)

Matrix+Debounce     Size (bytes)
---------------     ------------
original            17740
new + sym_g         17284
new + eager_pr      18106
new + eager_pk      18204

I expect that there are some scanning speed benefits as well.

* Keyboard: update frosty_flake's UNUSED_PINS

* Keyboard: Remove meaningless weak redefinitions from frosty

These are not necessary (and all of them already live somewhere in
Quantum).

keyboards/bpiphany/frosty_flake/rules.mk

@@ -30,7 +30,7 @@ BLUETOOTH_ENABLE = no       # Enable Bluetooth with the Adafruit EZ-Key HID
 AUDIO_ENABLE = no           # Audio output on port C6
 FAUXCLICKY_ENABLE = no      # Use buzzer to emulate clicky switches
 
-CUSTOM_MATRIX = yes
+CUSTOM_MATRIX = lite
 SRC += matrix.c
 
 LAYOUTS = tkl_ansi