[Keyboard] Add support for jj4x4 numpad/macropad by Kprepublic (#5016)

* added keyboard jj4x4, a shorter version of the jj40

* removed useless file

* edited jj4x4 readme.md

* optimized array size in jj4x4 config.h, removed reference to jj40 layouts from rules.mk

* removed custom matrix for this ps2avrgb board, refactored column and row pins

keyboards/jj4x4/keymaps/default/keymap.c

@@ -0,0 +1,34 @@
+#include QMK_KEYBOARD_H
+
+
+#define _QWERTY 0
+
+
+enum custom_keycodes {
+  QWERTY = SAFE_RANGE,
+};
+
+const uint16_t PROGMEM keymaps[][MATRIX_ROWS][MATRIX_COLS] = {
+/* Qwerty
+ * ,---------------------------.
+ * |   A  |   B  |   C  |  D   |
+ * +------+------+------+------|
+ * |   E  |   F  |   G  |  H   |
+ * +------+------+------+------|
+ * |   I  |   J  |   K  |  L   |
+ * +------+------+------+------|
+ * |   M  |   N  |   O  |  P   |
+ * ----------------------------'
+ */
+[_QWERTY] = LAYOUT( \
+  KC_A,    KC_B,    KC_C,   KC_D, \
+  KC_E,    KC_F,    KC_G,   KC_H, \
+  KC_I,    KC_J,    KC_K,   KC_L, \
+  KC_M,    KC_N,    KC_O,   KC_P  \
+),
+};
+
+// Loop
+void matrix_scan_user(void) {
+  // Empty
+};